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.

src/dsl/grammar/ddl.rs

@@ -18,7 +18,7 @@ use crate::dsl::command::{
 };
 use crate::dsl::value::Value;
 use crate::dsl::grammar::{
-    CommandNode, HintMode, IdentSource, Node, ValidationError, Word,
+    CommandNode, HighlightClass, HintMode, IdentSource, Node, ValidationError, Word,
     shared::{REFERENTIAL_CLAUSES, TYPE_SLOT, TYPE_VALIDATOR},
 };
 
@@ -1079,7 +1079,7 @@ const COL_SPEC_NODES: &[Node] = &[
         source: IdentSource::Types,
         role: "col_type",
         validator: Some(TYPE_VALIDATOR),
-        highlight_override: None,
+        highlight_override: Some(HighlightClass::Type),
         writes_table: false,
         writes_column: false,
         writes_user_listed_column: false,