ADR-0024 Phase F (full) step 3: delete legacy parser modules

Removes the last consumers of `dsl::lexer`, `dsl::keyword`, and
`dsl::ident_slot`, then deletes the modules.

- `Theme::token_color(&TokenKind)` deleted along with its test;
  `Theme::highlight_class_color(HighlightClass)` is the sole
  highlight-colour mapper (the walker's `per_byte_class` feeds
  it directly).
- `IdentSource` (`dsl::grammar`) absorbs the schema-list /
  expected-label / round-trip semantics that previously lived
  on `IdentSlot`. Adds `completes_from_schema`, `expected_label`,
  and `from_expected_label` methods. The walker's
  `Expectation::Ident { source }` and the schema-lookup request
  on the database worker now share one enum.
- `SchemaCache::for_slot(IdentSlot)` → `for_source(IdentSource)`.
- `Database::list_names_for` and the `Request::ListNamesFor`
  worker variant take `IdentSource`. Internal tables and column
  / relationship lookups dispatch on the same enum.
- `InvalidIdent.slot: IdentSlot` → `InvalidIdent.source: IdentSource`.
  The `invalid_ident_at_cursor` rendering branch in
  `input_render.rs::ambient_hint` updates accordingly.
- Completion's keyword filter (`Keyword::from_word`) becomes
  "backticked items whose payload is all ASCII alphabetic" —
  punct and digit literals still surface through their own
  candidate sources (composite-literal, flag, schema-ident);
  the alphabetic filter excludes them from the keyword bucket.
- `friendly::keys::tests::keyword_and_punct_have_complete_token_vocabulary`
  is dropped. It cross-checked `Keyword::ALL` / `Punct::ALL`
  against catalog entries; both enums are gone. The
  `parse.token.keyword.*` / `parse.token.punct.*` catalog
  entries themselves survive for one more commit (catalog
  cleanup, ADR-0024 §cleanup-pass); the
  `keys_validate_against_catalog` test still pins them.
- Modules deleted: `src/dsl/lexer.rs`, `src/dsl/keyword.rs`,
  `src/dsl/ident_slot.rs`.

Tests: 806 passing, 0 failing, 1 ignored. The drop from 852
reflects the removed module-internal tests (~32 lexer, 7
keyword, 4 ident_slot, 1 theme token_color, 1 friendly keys
keyword/punct), and is the expected outcome.

Clippy clean with `nursery` lints + `-D warnings`.

src/input_render.rs

@@ -210,15 +210,16 @@ pub fn ambient_hint(
     // the typed prefix matches nothing in the schema. (Stage
     // 8e / the user's #5.)
     if let Some(inv) = crate::completion::invalid_ident_at_cursor(input, cursor, cache) {
-        let kind = match inv.slot {
-            crate::dsl::ident_slot::IdentSlot::TableName => "table",
-            crate::dsl::ident_slot::IdentSlot::Column => "column",
-            crate::dsl::ident_slot::IdentSlot::RelationshipName => "relationship",
-            // `NewName` is filtered out by `invalid_ident_at_cursor`
-            // (it only fires for known-set slots), so this arm
-            // is unreachable in practice; render a neutral
-            // fallback rather than panic.
-            crate::dsl::ident_slot::IdentSlot::NewName => "identifier",
+        let kind = match inv.source {
+            crate::dsl::grammar::IdentSource::Tables => "table",
+            crate::dsl::grammar::IdentSource::Columns => "column",
+            crate::dsl::grammar::IdentSource::Relationships => "relationship",
+            // `NewName`, `Types`, `Free` are filtered out by
+            // `invalid_ident_at_cursor` (it only fires for
+            // known-set sources via `completes_from_schema`), so
+            // these arms are unreachable in practice — render a
+            // neutral fallback rather than panic.
+            _ => "identifier",
         };
         return Some(AmbientHint::Prose(crate::t!(
             "hint.ambient_invalid_ident",