feat: give column data types a dedicated syntax-highlight colour

Both Node::Ident and Word carried a highlight_override field, and
both were dead — the walker driver discarded the Ident's and
walk_word hardcoded Keyword. So column types (int, serial, …)
rendered identically to table/column names.

Wire both overrides through, and add a dedicated HighlightClass::Type
with its own theme colour (tok_type), distinct from keyword-purple
and identifier-teal. The three type Ident slots opt in, so canonical
types and the advanced-mode single-word SQL aliases (float, varchar,
…) render as types; the two-word `double precision` alias opts in via
a new Word::type_keyword constructor. ADR-0022 Amendment 4.

docs/adr/0022-ambient-typing-assistance.md

@@ -587,6 +587,61 @@ guard above, and three re-baselined `typing_surface` snapshots
 (`form_a_in_progress_one_value`, `form_b_too_few_values`,
 `form_c_wrong_count`).
 
+## Amendment 4 — Column types get a dedicated highlight class (2026-05-29)
+
+§3 introduced **seven** token-class colour fields and the matching
+`HighlightClass` enum. Column data-type keywords (`int`, `serial`,
+`text`, …) had no class of their own: the walker driver hardcoded
+`HighlightClass::Identifier` for every matched `Node::Ident`, so a
+type rendered identical to a table/column name (clause keywords were
+already distinct, in `tok_keyword`). A user-reported bug (issue #8)
+noted that in `create table Orders (count int, id serial PRIMARY KEY)`
+the identifiers and the type keywords were indistinguishably teal.
+
+Both terminal kinds already carried a `highlight_override:
+Option<HighlightClass>` field — `Node::Ident` and the `Word` struct
+alike — but **both were dead**: the driver destructured the Ident's to
+`_`, and `walk_word` hardcoded `Keyword`, neither ever consulting the
+field. This amendment wires both through.
+
+**Change:**
+
+1. **New class.** `HighlightClass::Type` (the eighth variant) and a
+   matching eighth `Theme` field `tok_type`, populated in both
+   `dark()` and `light()` with a tone deliberately distinct from both
+   `tok_keyword` and `tok_identifier` (a pink / deep-magenta in the
+   red-purple range — the only free slot in the existing palette).
+   `highlight_class_color` maps the new variant.
+2. **Overrides are now live.** `walk_ident` emits
+   `override.unwrap_or(Identifier)` and `walk_word` emits
+   `override.unwrap_or(Keyword)` for the matched byte range. Every
+   slot that leaves the field `None` is unchanged.
+3. **Type slots opt in.** The three `IdentSource::Types` slots —
+   `shared::TYPE_SLOT` (`add column` / `change column`), the inline
+   create-table column-type Ident (`ddl.rs`), and `sql_create_table::
+   SQL_TYPE_NAME` (advanced) — set `highlight_override: Some(Type)`.
+   In advanced mode, every single-word SQL type-name *alias*
+   (`float`, `varchar`, `integer`, … — ADR-0035 §3) flows through
+   `SQL_TYPE_NAME` and so is type-coloured for free.
+4. **`double precision` too.** The lone two-word alias (ADR-0035 §3)
+   is matched as keyword tokens, not an `IdentSource::Types` Ident, so
+   it cannot ride rule 3. Its `double` / `precision` grammar nodes use
+   the new `Word::type_keyword` constructor (`highlight_override:
+   Some(Type)`) so the spelling renders type-coloured like its
+   single-word synonyms `float` / `real`.
+
+**Pedagogy:** a dedicated colour (over the lighter option of reusing
+`tok_keyword`) lets a learner tell *"this is a type"* from a clause
+keyword and from a name they invented — three distinct roles, three
+distinct colours.
+
+**Coverage:** `dsl_type_keyword_classified_as_type`,
+`advanced_type_keywords_classified_as_type` (the ticket's exact
+example), `advanced_double_precision_classified_as_type`,
+`type_colour_is_distinct_from_keyword_and_identifier`, and the
+extended theme mapping/contrast tests. Text snapshots are colour-blind
+(`render_to_string` strips style), so none churned.
+
 ## Out of scope
 
 Deliberately deferred to keep this ADR shippable as a single