feat: ADR-0035 4d — CREATE [UNIQUE] INDEX / DROP INDEX

Advanced-mode SQL CREATE [UNIQUE] INDEX [IF NOT EXISTS] [<name>] ON
<T> (cols) -> SqlCreateIndex and DROP INDEX [IF EXISTS] <name> ->
SqlDropIndex, both reusing the ADR-0025 executors (do_add_index /
do_drop_index), like 4c reused do_drop_table.

- CREATE UNIQUE INDEX admitted in advanced mode (ADR-0025 Amendment 1):
  ADR-0025 deferred UNIQUE indexes for the simple-mode DSL, but advanced
  mode trusts the user like SQL does. Adds an additive IndexSchema.unique
  flag (project.yaml, serde-default, version stays 1); rebuild re-emits
  CREATE UNIQUE INDEX; the redundant-set guard keys on (columns, unique).
  Simple-mode `add unique index` stays deferred.
- IF [NOT] EXISTS on both forms reuses the 4c no-op-with-note skip
  (journalled, not snapshotted) via CreateIndexOutcome / DropIndexOutcome.
- Unnamed CREATE INDEX auto-named (ADR-0025 convention); the [UNIQUE]
  prefix is a concrete-keyword Choice and the optional name an on-led-first
  selector (the drop-index selector precedent) — trap-safe.
- create/drop each gain a second advanced node; the existing all-candidates
  dispatch handles it (locked by parse tests).
- Unique indexes marked [unique] in the structure view and items panel.
- do_add_index refuses internal __rdbms_* tables as "no such table",
  closing a latent exposure on both the simple `add index` and the new
  SQL CREATE INDEX surfaces (ADR-0025 Amendment 1).

Docs: ADR-0035 status + §13 4d + 4i; ADR-0025 Amendment 1; ADR README;
requirements.md Q1/C3. Plan: docs/plans/20260525-adr-0035-sql-ddl-4d.md.

Tests: 1834 passing / 0 failing / 0 skipped / 1 ignored; clippy clean.

docs/adr/0035-advanced-mode-sql-ddl.md

@@ -3,12 +3,13 @@
 ## Status
 
 Accepted. Design agreed with the user (2026-05-24); the approach is
-**validated end-to-end by sub-phases 4a / 4a.2 / 4a.3 / 4b / 4c**
+**validated end-to-end by sub-phases 4a / 4a.2 / 4a.3 / 4b / 4c / 4d**
 (`CREATE TABLE` with column- and table-level constraints and foreign
-keys, and `DROP TABLE [IF EXISTS]`, implemented 2026-05-25 — plans
+keys, `DROP TABLE [IF EXISTS]`, and `CREATE [UNIQUE] INDEX` /
+`DROP INDEX [IF EXISTS]`, implemented 2026-05-25 — plans
 `docs/plans/20260524-adr-0035-sql-ddl-4a.md`, `…-4a2.md`, `…-4a3.md`,
-`docs/plans/20260525-adr-0035-sql-ddl-4b.md`, `…-4c.md`), so the
-decision is accepted while the remaining sub-phases (**4d–4i**, §13)
+`docs/plans/20260525-adr-0035-sql-ddl-4b.md`, `…-4c.md`, `…-4d.md`), so
+the decision is accepted while the remaining sub-phases (**4e–4i**, §13)
 continue. This is **Phase 4** of the ADR-0030 roadmap (the
 advanced-mode SQL surface), the peer of ADR-0031 (expression grammar),
 ADR-0032 (`SELECT`), and ADR-0033 (DML). It **clarifies ADR-0030 §4**
@@ -381,7 +382,25 @@ ADR-0033's structure:
   DSL drops still parse via fallback — 4i grows the surface as `DROP
   INDEX` lands in 4d.
 - **4d — `CREATE [UNIQUE] INDEX` / `DROP INDEX`** → `SqlCreateIndex`
-  / `SqlDropIndex` (ADR-0025; the `UNIQUE` flag extension if needed).
+  / `SqlDropIndex`. *(Implemented 2026-05-25 — plan
+  `docs/plans/20260525-adr-0035-sql-ddl-4d.md`.)* Both reuse the ADR-0025
+  executors (`do_add_index` / `do_drop_index`), like 4c reused
+  `do_drop_table`. `CREATE [UNIQUE] INDEX [IF NOT EXISTS] [<name>] ON <T>
+  (cols)` (the unnamed form auto-named per ADR-0025; the leading
+  `[UNIQUE]` is a concrete-keyword `Choice`, the optional name an
+  `on`-led-first selector mirroring the `drop index` positional
+  selector) and `DROP INDEX [IF EXISTS] <name>` (name-only — the
+  positional `drop index on T(…)` stays the simple form via fallback).
+  `IF [NOT] EXISTS` reuses the 4c no-op-with-note skip (journalled, not
+  snapshotted). **`CREATE UNIQUE INDEX` is admitted** (user-confirmed
+  2026-05-25): ADR-0025 deferred UNIQUE indexes for the *simple-mode
+  DSL*, but advanced mode "trusts the user like SQL does" (§7) — so the
+  model gains an `IndexSchema.unique` flag (additive YAML, `version` 1;
+  rebuild re-emits `CREATE UNIQUE INDEX`; the structure view + items
+  panel mark `[unique]`), recorded as **ADR-0025 Amendment 1**.
+  Simple-mode `add unique index` stays deferred. `create`/`drop` each
+  gain a *second* advanced node, exercising the all-candidates dispatch
+  (`decide` tries every advanced candidate).
 - **4e — `ALTER TABLE` add/drop/rename column.** Drop/rename column
   must guard against a **table-level CHECK that references the column**
   (4a.3): today the rebuild rejects it cleanly via the engine (the
@@ -398,8 +417,13 @@ ADR-0033's structure:
   statement), `help`/usage for the new forms. **Carried in from earlier
   slices:** (a) refresh the `CREATE TABLE` help/usage skeleton for the
   4a.2 `DEFAULT`/`CHECK`/composite-`UNIQUE`, 4a.3 table-`CHECK`, and 4b
-  FK forms (deferred from each); (b) `describe` display of table-level
-  constraints (composite `UNIQUE` + table `CHECK`); (c) **4b self-ref
+  FK forms (deferred from each) — **4d's index forms already carry their
+  own help/usage** (`ddl.sql_create_index` / `ddl.sql_drop_index` + the
+  `parse.usage.*` keys), since the nodes are new; (b) `describe` display
+  of table-level constraints (composite `UNIQUE` + table `CHECK`) — note
+  the **unique-*index* marker shipped in 4d** (`[unique]` in the
+  structure view + items panel), so only the table-level *constraint*
+  display remains here; (c) **4b self-ref
   FK indicator** — a `CREATE TABLE` with a self-referencing FK
   (`references <self>`) parses + executes correctly, but the pre-submit
   schema-existence diagnostic falsely flags the not-yet-created self
@@ -415,7 +439,10 @@ ADR-0033's structure:
   entry word so advanced completion offers every valid continuation
   (`drop ` → table + column + relationship + index + constraint; `drop
   rel` → relationship); verify `create`/`insert`/`update`/`delete`
-  completion stays sensible. (e) **Discussion flag (user, 2026-05-25):**
+  completion stays sensible. **4d widened this:** `create` and `drop`
+  now each have *two* advanced nodes (table + index), so a shared entry
+  word's continuations now span two SQL shapes as well as the DSL ones —
+  the merge matters more. (e) **Discussion flag (user, 2026-05-25):**
   before/with (d), discuss **visually distinguishing simple- vs
   advanced-mode completions in the hint UI (likely by colour)** so a
   learner can see which continuations are DSL and which are SQL — a UX