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/dsl/parser.rs

@@ -180,27 +180,29 @@ fn punct<'a>(
 /// command parsers must use `ident_ctx(slot)` so the
 /// completion engine knows what kind of identifier each
 /// position expects (ADR-0022 §8). Bare `ident_inner()` calls
-/// outside this module would skip the slot annotation.
+/// outside this module would skip the slot annotation. The
+/// label is applied by `ident_ctx` (one per call site) — none
+/// here.
 fn ident_inner<'a>()
 -> impl Parser<'a, &'a [Token], String, extra::Err<Rich<'a, Token>>> + Clone {
     select_ref! {
         Token { kind: TokenKind::Identifier(s), .. } => s.clone()
     }
-    .labelled("identifier")
-    .as_context()
 }
 
-/// Tag-and-parse an identifier slot. `slot` is currently
-/// documentation-only — the parser does not propagate it to
-/// chumsky's `extra` data — but the call-site annotation
-/// forces every parser author to think about the slot at the
-/// moment of writing the combinator. The `no_bare_ident_inner_calls`
-/// unit test enforces that no command parser calls `ident_inner`
-/// directly (only via this wrapper).
+/// Tag-and-parse an identifier slot. The slot's user-facing
+/// label (`IdentSlot::expected_label`) replaces the generic
+/// "identifier" in the parser's expected-set machinery, so
+/// the error message reads "expected table name" /
+/// "expected column name" / "expected relationship name" /
+/// "expected identifier" depending on the call site
+/// (ADR-0022 stage 8c). The completion engine reverses the
+/// mapping via `IdentSlot::from_expected_label` to know what
+/// schema list to consult.
 fn ident_ctx<'a>(
-    _slot: crate::dsl::ident_slot::IdentSlot,
+    slot: crate::dsl::ident_slot::IdentSlot,
 ) -> impl Parser<'a, &'a [Token], String, extra::Err<Rich<'a, Token>>> + Clone {
-    ident_inner()
+    ident_inner().labelled(slot.expected_label()).as_context()
 }
 
 /// Match a number-literal token, returning a `Value::Number`.
@@ -912,9 +914,13 @@ mod tests {
 
     #[test]
     fn structural_error_for_show_data_without_arg() {
+        // ADR-0022 stage 8c: `ident_ctx(IdentSlot::TableName)`
+        // labels the expected slot with "table name" so the
+        // error reads as the more specific "expected table
+        // name" rather than the generic "expected identifier".
         let msg = err_message("show data");
         assert!(msg.contains("after `show data`"), "{msg}");
-        assert!(msg.contains("expected identifier"), "{msg}");
+        assert!(msg.contains("expected table name"), "{msg}");
         assert!(msg.contains("found end of input"), "{msg}");
     }