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/requirements.md

@@ -106,6 +106,10 @@ handoff-14 cleanup; 449 after B2/C2.)
 - [ ] **I3** Tab completion for app commands, DSL keywords, table
   names, column names, and SQL keywords.
 - [ ] **I4** Syntax highlighting for both the DSL and SQL.
+  *(Refinement 2026-05-29, issue #8: column data types now carry a
+  dedicated `HighlightClass::Type` / `tok_type` colour, distinct from
+  identifiers and clause keywords — ADR-0022 Amendment 4. The broad
+  highlighting goal stays open.)*
 - [ ] **I5** In-flight query/command cancellation (Ctrl-C in the
   output area or input field).