docs(adr): finalize ADR-0058 number for clause-concept-hints

Rename the placeholder draft to its reserved number (0058, allocated
reserve-first via the new dev-workflow flow), drop the ADR-XXXX
placeholders, and add the README index row.

docs/adr/0058-clause-concept-hints.md

@@ -0,0 +1,462 @@
+# ADR-0058: Clause-concept hints — a `hint.concept.*` layer surfaced by cursor position (issue #37)
+
+## Status
+
+Accepted — **implemented 2026-06-23** on branch `feat/clause-concept-hints`
+(pending merge). Number **0058**, reserved up front via the reserve-first
+flow (ADR-0059) rather than the older placeholder-until-merge default —
+this ADR is the first to carry a number stable from creation. Extends
+**ADR-0053** (contextual `hint`, D3/D4) additively; does not supersede it.
+Closes the deferred extension tracked as Gitea issue **#37**.
+
+Implementation: `Node::Concept { topic, inner }` (`dsl/grammar/mod.rs`) +
+the span-recording driver arm (`dsl/walker/driver.rs`) +
+`WalkContext::concept_spans` / `ConceptSpan` (`dsl/walker/context.rs`);
+the `concept_topic_at_cursor` resolver (`dsl/walker/mod.rs`); 10
+`Node::Concept` wrappers across `shared.rs` / `ddl.rs` /
+`sql_create_table.rs` (one shared `REFERENTIAL_CLAUSES`, two extracted
+cardinality sub-`Seq`s); the mode-keyed `emit_tier3_block` +
+`note_hint_for_input` layering (`app.rs`); the seven `hint.concept.*`
+blocks + `hint.block.concept_heading` (`strings/en-US.yaml`) with
+`keys.rs` declarations; and the test set (14 resolver unit tests, 3
+comprehensiveness gates incl. the recursive `Node::Concept` visitor, 3 F1
+integration tests, the `hint_block_with_concept` snapshot). Suite green
+across all tiers; `fmt`/`clippy` clean.
+
+Revised after a `/runda` review (2026-06-22) that grounded the mechanism
+against the code and caught two design gaps: the clause mapping (D4)
+covered only the advanced-SQL constraint grammar and missed the
+simple-mode constraint suffix (ADR-0029); and the single-`example`
+content model collided with ADR-0053 D6's mode-correct-example rule for
+topics reachable in both modes. Both are resolved here — D4 now maps both
+grammars, and D5 adopts **mode-keyed examples** (user decision,
+2026-06-22).
+
+References ADR-0053 (the three-tier `hint` model + the `hint.cmd.*` /
+`hint.err.*` corpus), ADR-0024 (the unified grammar/walker, `HintMode`-
+per-node, `MatchedPath` spans), ADR-0022 (ambient tier-2 typing
+assistance), ADR-0013/0045 (the relationship + `m:n` grammar), ADR-0035
+(advanced-mode `CREATE TABLE` constraint grammar).
+
+## Context
+
+ADR-0053 delivers tier-3 teaching hints at **per-form** granularity: one
+`hint.cmd.<form>` block (`what` / `example` / `concept`) per command form,
+surfaced by F1 on the live input. Position-awareness *within* a form is
+owned by tier-2 (the ambient candidate list / slot prose, ADR-0022).
+
+During ADR-0053 Phase B we identified a third teaching point the per-form
+model does not serve: **concepts that live inside a clause**, where a
+learner may want teaching deeper than tier-2's candidate list but narrower
+than the whole-form block. The canonical examples are the most
+load-bearing relational concepts in the tool:
+
+- the **referential actions** clause — `… on delete ⟨cascade | set null |
+  restrict | no action⟩` (and `on update`);
+- the **cardinality** marker — `1:n` (one parent, many children) and
+  `m:n` (many-to-many, auto-junction per ADR-0045);
+- the **primary-key** declaration — `with pk(…)` in simple mode and the
+  `primary key` constraint in `create table (…)`;
+- the **column-constraint slots** in `create table (…)` — `unique`,
+  `check (…)`, `foreign key` / `references`.
+
+A student parked in one of these clauses is, by definition, looking at the
+exact relational concept that clause embodies. ADR-0053 explicitly
+deferred this ("clause-concept hints", issue #37) and recorded that the
+per-form keying does **not** lock it out: a later `hint.concept.<topic>`
+namespace can be surfaced when the cursor sits in a recognized clause,
+layered on top of the per-form block.
+
+This ADR is that follow-up. It is **pedagogy-led, not metrics-led** — we
+do not expect to accumulate usage statistics about which clauses learners
+pause at, so the clause set is chosen by relational-teaching value, not
+data.
+
+### Two user decisions taken up front (2026-06-22)
+
+The issue left two scope questions open. Both were decided by the user
+before design:
+
+1. **Detection precision: "anywhere inside the clause."** The hint fires
+   whenever the cursor sits *within* a recognized clause's span —
+   including parked inside already-typed text (e.g. `add 1:n
+   relation|ship …`) — not only at the slot boundary where the next token
+   is about to be typed.
+2. **Clause scope: all four families** — referential actions, the
+   create-table constraint slots, `with pk`, and `1:n` / `m:n`
+   cardinality.
+
+### Why the existing `HintMode`/`pending_hint_mode` mechanism is not enough
+
+Tier-2 slot prose is driven by `Node::Hinted { mode, inner }`: entering
+the node sets `ctx.pending_hint_mode`, and the resolver reads it at the
+end of the cursor-trimmed walk (`hint_resolution_at_input_in_mode`,
+`src/dsl/walker/mod.rs:107`). Crucially, the driver **clears
+`pending_hint_mode` on every successful match** (`src/dsl/walker/
+driver.rs:154`): it records only "the Hinted slot the cursor is *about to
+fill*", and the moment the slot's content matches, the signal is gone.
+
+That is exactly the *boundary* semantics tier-2 wants — and exactly the
+opposite of "anywhere inside the clause". The instant the learner finishes
+typing `cascade`, `pending_hint_mode` clears, so a cursor parked on the
+finished `on delete cascade` would see nothing. Decision (1) above
+therefore needs a **span-based** mechanism that survives the clause being
+fully matched, not the single-slot `pending_hint_mode`.
+
+## Decision
+
+### D1 — A span-recording grammar wrapper, `Node::Concept`
+
+Add a new grammar combinator, sibling to `Node::Hinted`:
+
+```rust
+Node::Concept { topic: &'static str, inner: &'static Node }
+```
+
+It is a pass-through for matching (it walks `inner` and returns `inner`'s
+result verbatim — it never changes what parses), but as a side effect the
+driver records the **byte span the node covered** into a new context
+field:
+
+```rust
+// WalkContext
+pub concept_spans: Vec<ConceptSpan>,   // { topic: &'static str, start: usize, end: usize }
+```
+
+- `start` = the first non-whitespace byte the node began matching at
+  (post `skip_whitespace`, matching the driver's existing convention).
+- `end` = the byte position the node's walk reached — the end of the
+  matched span on a full match, or the stop position on an incomplete /
+  failed inner. This makes the span the real extent of the clause as typed
+  so far.
+
+Unlike `pending_hint_mode`, `concept_spans` is **append-only across the
+whole walk** and is never cleared on match — so a fully-typed clause keeps
+its span, satisfying decision (1).
+
+**Precise push rule** (grounded in `NodeWalkResult`,
+`src/dsl/walker/driver.rs:97`): walk `inner`, then push `{ topic, start,
+end }` iff the inner result is one of `Matched { end }` (end = `end`),
+`Incomplete { position }`, or `Failed { position }` (end = `position`) —
+i.e. the node **committed** (consumed at least its first token, or ran out
+mid-clause). On `NoMatch { .. }` (the node never engaged — e.g. a `Choice`
+tried this branch and it didn't apply) push **nothing**. This is the
+`NodeWalkResult`-level analogue of the driver's existing
+`pending_hint_mode` clear-on-match leak-avoidance, and it means a failed
+`Choice` branch leaves no stale span.
+
+`Node::Concept` is **orthogonal to `Node::Hinted`** — a clause that wants
+both a tier-2 slot prose *and* a tier-3 concept span nests them (`Concept{
+… inner: Hinted{ … } }`). Neither changes the parse.
+
+### D2 — Cursor → topic resolution: innermost containing span
+
+A new resolver:
+
+```rust
+pub fn concept_topic_at_cursor(
+    input: &str, cursor: usize,
+    schema: Option<&SchemaCache>, mode: Mode,
+) -> Option<&'static str>
+```
+
+walks the **full** input buffer with `WalkBound::EndOfInput` (the dormant
+`WalkBound::Position` path is *not* used — see D3 rationale), collects
+`ctx.concept_spans`, and returns the `topic` of the **innermost span
+containing `cursor`**. "Innermost" = the containing span with the latest
+`start` (equivalently, the narrowest), so when a `references` clause
+(constraint slot) wraps a referential-actions clause, a cursor on `on
+delete` resolves to the more specific `referential_actions`, while a
+cursor on `references Parent` resolves to `foreign_key`.
+
+Containment is inclusive of `start` and `end` (`start <= cursor <= end`),
+so the boundary case — cursor *at* the point of entering the clause —
+also resolves, meaning this subsumes the simpler boundary detection for
+free.
+
+The `cursor` passed in is the one **already computed by `feedback_view()`**
+(`src/app.rs:3204` returns `(view, cursor, _off)`), which strips the `:`
+one-shot sigil and adjusts the cursor offset to index into the stripped
+`view`. The concept resolver walks that same `view`, so the `:`-strip is
+handled with no extra offset arithmetic.
+
+### D3 — Why full-buffer span-containment, not `WalkBound::Position`
+
+`WalkBound::Position(cursor)` (defined, `#[allow(dead_code)]`,
+`src/dsl/walker/outcome.rs:26`) slices the source at the cursor and walks
+the prefix. Two reasons it is the wrong tool here:
+
+- **It discards text after the cursor.** A cursor parked early in a
+  complete command (`add 1:n| relationship from … on delete cascade`)
+  would truncate to `add 1:` and lose the parsed structure that tells us
+  we are inside a well-formed relationship command. Full-buffer walking
+  keeps the whole parse; the cursor is used *only* for span containment.
+- **It still rides `pending_hint_mode`'s clear-on-match**, which D1
+  already establishes is unsuitable for inside-clause detection.
+
+Span-recording over a full `EndOfInput` walk needs no new walk-bound
+behaviour and leaves the dormant `Position` path untouched.
+
+### D4 — The clause set and their topics (both grammars)
+
+**Seven** `hint.concept.<topic>` blocks, placed via `Node::Concept`
+wrappers across **both** the simple-mode (`grammar/ddl.rs`, ADR-0029) and
+advanced-mode (`grammar/sql_create_table.rs`, ADR-0035) grammars. The
+codebase has **two distinct constraint grammars** — simple-mode `create
+table` column constraints (`COLUMN_CONSTRAINT_SUFFIX`, `ddl.rs:1082`:
+`not null` / `unique` / `default` / `check`; PK is `with pk`, FK is a
+relationship) and the advanced-SQL constraint set (`COL_CONSTRAINT_CHOICES`,
+`sql_create_table.rs`: adds `primary key` / `references`). A topic
+reachable in both modes is wrapped in **both** grammars (or once, if the
+node is physically shared — e.g. `REFERENTIAL_CLAUSES` in `shared.rs` is
+reused by the relationship command *and* the advanced `references` clause).
+
+| Topic | Modes | Grammar node(s) to wrap |
+|---|---|---|
+| `referential_actions` | both | `REFERENTIAL_CLAUSES` (`shared.rs` — one wrapper, reused in both) |
+| `cardinality_one_to_many` | simple only | a new `CARDINALITY_1N` sub-`Seq` extracted from `ADD_RELATIONSHIP_NODES` (`ddl.rs:479`) |
+| `cardinality_many_to_many` | simple only | a new `CARDINALITY_MN` sub-`Seq` extracted from `CREATE_M2N` head (`ddl.rs`) |
+| `primary_key` | both | `WITH_PK` (`ddl.rs`, simple) + the `primary key` constraint node (`sql_create_table.rs`, advanced) |
+| `unique` | both | the `unique` constraint node in `ddl.rs` (`UNIQUE_CONSTRAINT`, `:1051`) + in `sql_create_table.rs` |
+| `check` | both | the `check (…)` constraint node in `ddl.rs` (`:1064`) + in `sql_create_table.rs` |
+| `foreign_key` | advanced only | `REFERENCES_CLAUSE` (`sql_create_table.rs:206`) |
+
+If a `unique` / `check` node turns out to be physically shared between the
+two grammars (as `REFERENTIAL_CLAUSES` is), one wrapper suffices — the
+mode-keyed example (D5) is selected by the **live mode** at F1 time, not by
+which grammar node matched, so wrapper-sharing never affects correctness.
+The exact wrapper count is pinned during implementation; the *topic* set is
+the seven above.
+
+**Nesting is intentional.** `foreign_key`'s `REFERENCES_CLAUSE` contains
+`referential_actions`' `REFERENTIAL_CLAUSES`, so a cursor on `references
+Parent` resolves to `foreign_key` and a cursor on the inner `on delete`
+resolves to `referential_actions` — D2's innermost-wins handles it.
+
+**Deliberate omissions** (faithful to issue #37's explicit list of
+"primary key | unique | check | foreign key"): the **`not null`** and
+**`default`** column constraints get **no** concept block in v1. They are
+mechanical rather than the headline relational concepts #37 targets;
+adding them later is purely additive (a wrapper + a block + a coverage
+entry).
+
+Cardinality is **split** (`_one_to_many` vs `_many_to_many`) rather than a
+single "cardinality" block: the two are pedagogically distinct — `m:n`
+auto-creates a junction table (ADR-0045) — the cursor is always
+unambiguously in one or the other, and both are **simple-mode-only**
+(`1:n`/`m:n` are app syntax, not SQL), so each carries a single plain
+example with no mode conflict.
+
+`referential_actions` is one block covering both `on delete` and `on
+update` and all four actions (the action set is the concept; the block
+explains the choice rather than each action in isolation).
+
+### D5 — The content model: `what` / `example` / `concept`, with mode-keyed examples
+
+Each `hint.concept.<topic>` block keeps the ADR-0053 D3 three-part shape —
+`what` / `example` / `concept`. The one extension is the **example**: to
+honour ADR-0053 D6's "mode-correct example, no cross-mode sharing" rule
+for topics reachable in both modes (the `/runda` finding), a both-mode
+topic carries **two** examples keyed by mode; a single-mode topic carries
+one plain example.
+
+```yaml
+# both-mode topic — mode-keyed example
+hint.concept.referential_actions:
+  what: "Decide what happens to child rows when the parent they point at is deleted, or its key changes."
+  example:
+    simple: "add 1:n relationship from Customers.id to Orders.customer_id on delete cascade"
+    advanced: "create table Orders (id int primary key, customer_id int references Customers(id) on delete cascade)"
+  concept: "A foreign key forbids orphans by default (restrict). `cascade`
+    deletes the children with the parent; `set null` keeps them but clears
+    the link. The action encodes a real rule about your data."
+
+# single-mode topic — one plain example
+hint.concept.cardinality_one_to_many:
+  what: "Link two tables so one parent row can own many child rows."
+  example: "add 1:n relationship from Customers.id to Orders.customer_id"
+  concept: "‘1:n’ is one parent, many children. The child table holds the
+    foreign key column pointing back at the parent — that single shared
+    value is what groups a parent's children together."
+```
+
+`what`/`example` are always present; `concept` is the teaching payload
+(it is what makes this tier-3). The both-mode topics are
+`referential_actions`, `primary_key`, `unique`, `check`; the single-mode
+(simple-only) topics are `cardinality_one_to_many`,
+`cardinality_many_to_many`; `foreign_key` is single-mode (advanced-only).
+
+The blocks live under the existing `hint:` namespace in
+`src/friendly/strings/en-US.yaml`, alongside `hint.cmd.*` / `hint.err.*`,
+and every leaf key is declared in
+`src/friendly/keys.rs::KEYS_AND_PLACEHOLDERS` (the validator requires
+exhaustive declaration — no prefix exemption for the `hint.*` families).
+For both-mode topics that means `hint.concept.<topic>.example.simple` +
+`.example.advanced`; for single-mode topics, `hint.concept.<topic>.example`.
+
+**Authoring constraints** (project copy rules — see `CLAUDE.md`): concept
+text is user-facing, so it must **never name the database engine** and
+must **not say "DSL"** — use "simple mode" / "advanced mode". The `concept`
+prose teaches the relational idea in plain language; examples are
+copyable, mode-correct lines.
+
+### D6 — Rendering: the concept block layers under the form block
+
+This is an **additive** F1 surface. When F1 fires on live input
+(`note_hint_for_input`, `src/app.rs:3200`):
+
+1. Resolve and emit the per-form `hint.cmd.<form>` block exactly as today
+   (unchanged).
+2. **Then** call `concept_topic_at_cursor(buffer, cursor, …)`. If it
+   yields a topic and `hint.concept.<topic>.what` exists, emit a second
+   tier-3 block beneath the form block.
+
+The two blocks are visually distinguished by their **heading**: the form
+block keeps the existing `hint.block.heading` ("Hint"); the concept block
+carries a distinct sub-heading from a new `hint.block.concept_heading`
+(e.g. "About this clause") so the learner sees the form-level help *and*
+the clause-level concept without confusing the two.
+
+`emit_tier3_block` gains **two** small parameters: the heading key, and an
+optional mode used to resolve the example. Its example lookup becomes:
+prefer `<stem>.example.<mode>` (where `<mode>` is `simple`/`advanced` from
+the live `effective_mode`), else fall back to a plain `<stem>.example`.
+This keeps a single renderer for all three tier-3 families — `hint.cmd.*`
+and `hint.err.*` keep a plain `.example` (lookup falls straight through),
+while `hint.concept.*` both-mode topics resolve the mode-keyed variant.
+The mode-correct-example contract of ADR-0053 D6 is thereby honoured by
+the *content keying* + this lookup, not by duplicating the renderer.
+
+The concept block is **never shown alone** and **never replaces** the form
+block — consistent with the issue ("layered on top of … does not
+replace"). If the per-form resolution fails (rare; falls back to tier-2),
+the concept layer is skipped too — F1 keeps a single coherent behaviour.
+
+The submitted `hint` command (last-error route, D2 of ADR-0053) is
+**unchanged** — it has no cursor, so concept hints do not apply there.
+
+The rendered shape of a form-block-plus-concept-block is locked by a new
+`insta` snapshot (`hint_block_with_concept`).
+
+### D7 — Comprehensiveness, validation, fallback
+
+- **Validation (build/test gate):** every `hint.concept.<topic>` leaf key
+  is declared in `keys.rs`; the existing `keys_validate_against_catalog`
+  test fails on a typo or an undeclared key, in either direction.
+- **Comprehensiveness coverage test:** extend the ADR-0053
+  comprehensiveness test with a `CONCEPT_TOPICS` table — each entry a
+  `(topic, ModeCoverage)` where `ModeCoverage` is `Both` or
+  `Single` — asserting: (a) every topic resolves to `hint.concept.<topic>`
+  with a `.what` (and, per its `ModeCoverage`, either `.example.simple`
+  +`.example.advanced` for `Both`, or a plain `.example` for `Single`);
+  and (b) every `Node::Concept` wrapper reachable from the REGISTRY
+  carries a topic in the table. (b) needs a **recursive `Node`-tree
+  visitor** — `Node` is a recursive enum (`Seq(&[Node])` / `Choice` /
+  `Optional` / `Repeated` / `Hinted` / `Concept`), so the test walks each
+  REGISTRY shape collecting `Node::Concept` topics. A wrapper added
+  without content (or a topic with a wrapper missing) fails the test —
+  this is what makes the clause set enforceable rather than aspirational,
+  since `keys.rs` only checks that *referenced* keys resolve.
+- **Graceful degrade:** `emit_tier3_block` already returns `false` when a
+  `.what` key is absent, so a missing concept block degrades to "form
+  block only" — never a blank or an error. The validation above means
+  that path is a safety net, not the plan.
+
+## Forks
+
+- **Detection precision = anywhere inside the clause** (user, 2026-06-22)
+  rather than slot-boundary-only. Drives the span-recording mechanism
+  (D1) over the simpler `pending_hint_mode` reuse.
+- **Clause scope = all four families** (user, 2026-06-22) rather than the
+  issue's "start with referential actions + constraint slots" subset.
+- **Mechanism = full-buffer span-containment, not `WalkBound::Position`**
+  (D3) — keeps the whole parse and leaves the dormant cursor-bound path
+  untouched. *Engineering choice; rationale in D3.*
+- **Mode-keyed examples for both-mode topics** (user, 2026-06-22,
+  resolving the `/runda` finding) rather than splitting into
+  `_simple`/`_advanced` blocks — one concept block per topic, two examples
+  selected by live mode (D5/D6). Honours ADR-0053 D6's mode-correct-example
+  rule without duplicating concept prose.
+- **Cardinality split into two blocks** (D4) rather than one — the `1:n`
+  and `m:n` concepts diverge (junction table). *Pedagogy choice.*
+- **`primary_key` shared between `with pk` and the constraint slot** (D4)
+  — one concept, two syntaxes (a mode-keyed example per D5). *Content
+  choice.*
+- **Distinct sub-heading for the concept block** (D6) rather than merging
+  into one block or an undifferentiated second "Hint" — keeps form-level
+  and clause-level guidance legible. *UX choice.*
+
+## Consequences
+
+- **A fourth contextual surface exists** within tier-3: clause-level
+  teaching, deeper than tier-2's candidate list, narrower than the
+  per-form block, surfaced by cursor position on F1.
+- **One new grammar node** (`Node::Concept`), **one new `WalkContext`
+  field** (`concept_spans`), **one new resolver**
+  (`concept_topic_at_cursor`), **two new renderer parameters** (heading
+  key + example mode), and **~10 `Node::Concept` wrappers** across both
+  constraint grammars (exact count pinned at implementation per D4's
+  shared-node note), plus **two extracted cardinality sub-`Seq` nodes**.
+  No change to what parses; no change to tier-2; no change to the
+  submitted `hint` path.
+- **A new content sub-namespace** — `hint.concept.*` (seven topics; the
+  four both-mode topics carry two examples each) — enters the catalogue
+  under `hint:`, validated by `keys.rs` and the comprehensiveness test.
+  Ongoing surface: a future clause worth teaching ships its
+  `Node::Concept` wrapper + block + coverage entry together (a checklist
+  item for feature ADRs that add clauses).
+- **CHANGELOG:** a new `[Unreleased] → Added` bullet (user-facing F1
+  behaviour), phrased under the copy rules (no engine name; "simple
+  mode"/"advanced mode", not "DSL").
+- **Testing:** Tier-1 unit tests for `concept_topic_at_cursor` across each
+  clause (cursor at boundary, cursor inside typed text, cursor outside →
+  `None`, nested `references`→`on delete` innermost-wins, both modes);
+  the F1 layering logic (form block always; concept block appended iff a
+  topic resolves; concept skipped when the form block falls back); the
+  `:` one-shot strip carried through (it must still resolve advanced
+  forms); Tier-2 `insta` snapshot `hint_block_with_concept`; Tier-3
+  integration tests (type a partial relationship command, move the cursor
+  into `on delete`, F1 → form block + referential-actions concept block,
+  **buffer / cursor / completion memo untouched**); the comprehensiveness
+  + `keys.rs` validation tests (D7).
+
+## Out of scope
+
+- **Concept hints on the submitted `hint` command** — OOS: no cursor; the
+  last-error route is unchanged (D6).
+- **A concept block for every clause in the grammar** — OOS for v1: the
+  eight wrappers cover the highest-value relational concepts (D4). More
+  can be added additively later, gated by the comprehensiveness test.
+- **Surfacing concept text in the always-on tier-2 ambient panel** — OOS
+  (rejected): tier-2 stays terse by design (ADR-0022); the concept layer
+  is on-demand via F1, consistent with all of tier-3.
+- **A `hint.concept.<topic>` reachable by an explicit argument** (e.g.
+  `hint cascade`) — OOS: ADR-0053 D1 keeps `hint` argument-free; `help
+  <topic>` owns explicit reference lookup.
+- **Localisation beyond `en-US`** — OOS (deferred): the catalogue is i18n-
+  structured (ADR-0019) but English-only for v1 (requirements X2).
+
+## Content inventory (implementation tracking)
+
+Seven `hint.concept.<topic>` blocks, authored to the ADR-0053 D7 voice
+(`what` / `concept` always; example(s) per mode coverage):
+
+- [x] `referential_actions` *(both — 2 examples)* — on delete/update;
+      cascade / set null / restrict / no action.
+- [x] `cardinality_one_to_many` *(simple — 1)* — one parent owns many
+      children; the FK lives on the child.
+- [x] `cardinality_many_to_many` *(simple — 1)* — junction table; ADR-0045
+      auto-creation.
+- [x] `primary_key` *(both — 2)* — uniquely identifies a row; shared by
+      `with pk` (simple) and the `create table` constraint (advanced).
+- [x] `unique` *(both — 2)* — no two rows share the value; distinct from a
+      PK.
+- [x] `check` *(both — 2)* — a per-row rule enforced on write.
+- [x] `foreign_key` *(advanced — 1)* — a child column promising to point
+      at a real parent.
+
+Implementation artifacts: the `Node::Concept` wrappers (D4 table, ~10
+across both grammars + the two extracted cardinality sub-`Seq`s); the
+`CONCEPT_TOPICS` coverage table (topic + `ModeCoverage`); the `keys.rs`
+declarations (mode-keyed example leaves for the four both-mode topics);
+the `hint_block_with_concept` snapshot; the CHANGELOG bullet.