ADR-0022 stage 8c: IdentSlot propagation + SchemaCache API

`IdentSlot` gains `expected_label()` and the round-trip
`from_expected_label()`. The four slot kinds map to the
user-facing labels "identifier" (NewName), "table name",
"column name", "relationship name".

`ident_ctx(slot)` now actually applies `slot.expected_label()`
as the chumsky label (was documentation-only after stage 6).
Parser errors and the hint panel's "expected: …" prose now
read with the slot-specific name: "expected table name"
instead of the generic "expected identifier". One parser
test updated accordingly; the four catalog `parse.token.*`
keys are unaffected (the slot labels are a parallel surface).

New `completion::SchemaCache { tables, columns,
relationships }` struct + `for_slot(slot) -> &[String]`
accessor. Empty by default; runtime wiring lands in a
follow-on substage. NewName slots return `&[]`
unconditionally.

`candidates_at_cursor` extended to accept `&SchemaCache`:
when the parser's expected-set includes a slot label,
schema candidates from the cache are added alongside the
keyword candidates. Both sources are then prefix-filtered,
combined, sorted, deduplicated. App::schema_cache field
threaded into both the App-side completion paths and the
ambient_hint computation in ui.

Tests: 738 passing, 0 failing, 1 ignored (730 baseline →
+8: 2 IdentSlot label round-trip tests, 6 completion-with-cache
cases covering table/column/relationship slots, prefix
filtering, empty cache, and NewName-no-candidates).
Clippy clean.

User-visible: identifier completion infrastructure is in
place but the cache is always empty — runtime wiring (the
next substage) will populate it on project load and after
successful DDL, at which point Tab on identifier slots
starts offering schema names.

src/ui.rs

@@ -696,6 +696,7 @@ fn render_hint_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rect
             &app.input,
             app.input_cursor,
             app.last_completion.as_ref(),
+            &app.schema_cache,
         ),
         EffectiveMode::AdvancedPersistent | EffectiveMode::AdvancedOneShot => None,
     };