ADR-0019 §6: runtime enrichment + row pinpointing

Closes the placeholder-substitution gap reported during manual
testing: FK violations were rendering `<value>` and `<column>`
literally because the App had no schema awareness. With this
change the runtime resolves the schema-dependent facts before
the App ever sees the failure.

## Architecture

- **Database** gains two public methods backed by new worker
  Request variants:
  - `read_relationships(table)` → (outbound, inbound) FK list
    (lifts the previously-private `read_relationships_*` pair
    into the public surface, behind a `RelationshipsReply`
    type alias).
  - `find_rows_matching(table, column, value, limit)` →
    `DataResult` for row pinpoint queries.

- **friendly module** gets:
  - New `FailureContext` struct: schema-resolved facts the
    runtime builds (table, column, value, parent_table,
    parent_column, child_table, optional diagnostic_table).
  - `TranslateContext` loses its lifetime parameter and gains
    `parent_table` / `parent_column` fields. All string fields
    are now `Option<String>` for ownership simplicity.
  - `TranslateContext::from_facts(operation, verbosity, facts)`
    helper.
  - Translator's FK paths now use `ctx.parent_table` /
    `ctx.parent_column` for child-side wording; FK Update gets
    a dedicated `fk_child_side_update` arm.
  - FK dispatch is enrichment-driven first
    (`parent_table` set → child-side; `child_table` set →
    parent-side), with operation as the tiebreaker.
  - The translator forwards `ctx.diagnostic_table` onto the
    `FriendlyError` so pinpointed rows render through the
    existing ADR-0017 §7 bordered renderer.

- **Event** `DslFailed` carries `(command, error, facts)`.
  The runtime populates `facts` via `enrich_dsl_failure`
  before posting the event.

- **Runtime** `enrich_dsl_failure(database, command, error)`
  classifies and resolves:
  - UNIQUE INSERT/UPDATE: parses `T.col` from engine message,
    finds the user's attempted value (with schema fallback
    for natural-order multi-value INSERT — including the
    serial/shortid auto-skip rule from `do_insert`), pinpoints
    the existing conflicting row(s) via `find_rows_matching`
    and renders as a `DiagnosticTable`.
  - NOT NULL INSERT/UPDATE: parses `T.col`; no value
    (definitionally null) and no pinpoint (engine doesn't
    identify the row).
  - FK INSERT/UPDATE: outbound relationship lookup picks the
    FK column the user is touching; resolves
    `parent_table`/`parent_column`/`value`. UPDATE falls back
    to inbound (parent-side) when no outbound match.
  - FK DELETE: inbound relationship lookup picks a child_table
    that references this row.

- **App** drops its old `attempted_value_for` /
  `column_from_qualified_target` helpers (their work moved to
  runtime where the Database is in scope).
  `build_translate_context` combines the runtime-supplied
  facts with the operation derived from the Command and the
  App's verbosity.

## Manual-test fixes folded in

Two issues surfaced during manual testing of the initial
implementation, both fixed:

1. Natural-order multi-value INSERT
   (`insert into Orders values (4, 11.99)`) skipped FK
   enrichment because `user_value_for_column` only knew the
   single-value short form. The schema-aware lookup
   (`user_value_for_column_with_schema`) now mirrors
   `do_insert`'s position-mapping rule (auto-generated
   columns skipped), so positional INSERTs onto tables with
   serial/shortid PKs resolve correctly. Regression test:
   `enrich_fk_insert_natural_order_multi_value_resolves_via_schema`.

2. The arity error on INSERT now lists the columns it
   expected — `expected 3 value(s) for (id, Name, Email), got 2`
   instead of the bare count. Surfaces what the user needs
   to fix without making them go check the schema.

## Tests

`tests/friendly_enrichment.rs` (+8 integration tests):
- UNIQUE INSERT with explicit columns: facts.{table, column,
  value, diagnostic_table} all resolved; pinpoint shows
  conflicting row.
- UNIQUE INSERT natural-order short form: schema fallback
  resolves the value.
- UNIQUE UPDATE: value pulled from assignments.
- NOT NULL INSERT: table+column resolved, value None
  (correct), no pinpoint.
- FK INSERT: parent_table, parent_column, value all resolved
  via outbound relationship lookup.
- FK INSERT natural-order multi-value: schema-aware lookup
  with auto-skip resolves correctly (regression for the
  manual-test bug).
- FK DELETE: child_table resolved via inbound relationship
  lookup.
- DbError::Unsupported: enrichment returns default
  FailureContext (no false positives).

App-level tests updated to populate `FailureContext` directly
(simulating runtime enrichment) for the verbosity / threading
checks.

## Tally

610 tests passing (was 603: +8 enrichment integration tests
minus 1 obsolete App-side helper test that the runtime
absorbed). Clippy clean with nursery lints. Release builds.

src/friendly/mod.rs

@@ -35,7 +35,7 @@ pub mod translate;
 
 pub use error::{DiagnosticTable, FriendlyError};
 pub use format::{catalog, Catalog};
-pub use translate::{Operation, TranslateContext, Verbosity};
+pub use translate::{FailureContext, Operation, TranslateContext, Verbosity};
 
 // `translate::translate` and `format::translate` are different
 // callables — the former is the structured DbError → FriendlyError