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/sql_create_table.rs

@@ -24,7 +24,9 @@
 //! `sql_insert::SQL_INSERT_SHAPE`, which starts at `INTO`).
 
 use crate::dsl::grammar::sql_select::reject_internal_table;
-use crate::dsl::grammar::{IdentSource, Node, ValidationError, Word, shared, sql_expr};
+use crate::dsl::grammar::{
+    HighlightClass, IdentSource, Node, ValidationError, Word, shared, sql_expr,
+};
 use crate::dsl::types::Type;
 
 static COMMA: Node = Node::Punct(',');
@@ -60,7 +62,7 @@ const SQL_TYPE_NAME: Node = Node::Ident {
     source: IdentSource::Types,
     role: "col_type",
     validator: Some(validate_sql_type_name),
-    highlight_override: None,
+    highlight_override: Some(HighlightClass::Type),
     writes_table: false,
     writes_column: false,
     writes_user_listed_column: false,
@@ -86,8 +88,8 @@ const LENGTH_OPT: Node = Node::Optional(&Node::Seq(LENGTH_NODES));
 // on its own (ADR-0035 §6.3). The builder maps the pair to
 // `Type::Real`.
 static DOUBLE_PRECISION_NODES: &[Node] = &[
-    Node::Word(Word::keyword("double")),
-    Node::Word(Word::keyword("precision")),
+    Node::Word(Word::type_keyword("double")),
+    Node::Word(Word::type_keyword("precision")),
 ];
 static TYPE_WITH_LENGTH_NODES: &[Node] = &[SQL_TYPE_NAME, LENGTH_OPT];
 static SQL_TYPE_CHOICES: &[Node] = &[