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/app.rs

@@ -135,6 +135,11 @@ pub struct App {
     /// keystroke — no completion mode, just a transient
     /// memory of "the last thing Tab did."
     pub last_completion: Option<crate::completion::LastCompletion>,
+    /// Per-project schema lookup cache feeding Tab completion
+    /// for identifier slots (ADR-0022 §9 + stage 8c). Empty
+    /// by default; refreshed by the runtime on project load
+    /// and after successful DDL.
+    pub schema_cache: crate::completion::SchemaCache,
 }
 
 /// Dialogs that take over keyboard input when active.
@@ -245,6 +250,7 @@ impl App {
             fatal_message: None,
             modal: None,
             last_completion: None,
+            schema_cache: crate::completion::SchemaCache::default(),
         }
     }
 
@@ -683,7 +689,11 @@ impl App {
     /// candidates to insert.
     fn start_completion(&mut self, start_idx: usize) -> Option<crate::completion::LastCompletion> {
         let cursor = self.input_cursor.min(self.input.len());
-        let comp = crate::completion::candidates_at_cursor(&self.input, cursor)?;
+        let comp = crate::completion::candidates_at_cursor(
+            &self.input,
+            cursor,
+            &self.schema_cache,
+        )?;
         let idx = start_idx % comp.candidates.len();
         let inserted = format!("{} ", comp.candidates[idx]);
         let original_text =
@@ -708,7 +718,11 @@ impl App {
         // before we can pick "last". Compute the completion
         // first, then call start_completion with that index.
         let cursor = self.input_cursor.min(self.input.len());
-        let comp = crate::completion::candidates_at_cursor(&self.input, cursor)?;
+        let comp = crate::completion::candidates_at_cursor(
+            &self.input,
+            cursor,
+            &self.schema_cache,
+        )?;
         let last = comp.candidates.len() - 1;
         // Fall through to the same path so the inserted text
         // and memo construction stay in one place.