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,

src/dsl/grammar/mod.rs

@@ -47,6 +47,12 @@ use crate::dsl::walker::outcome::MatchedPath;
 pub enum HighlightClass {
     Keyword,
     Identifier,
+    /// Column data-type keyword (`int`, `serial`, `text`, …).
+    /// Distinct from `Keyword` and `Identifier` so learners can
+    /// tell "this is a type" from a clause keyword or a name they
+    /// invented (ADR-0022 Amendment 4). Assigned via a type slot's
+    /// `highlight_override`, not by byte shape.
+    Type,
     Number,
     String,
     Punct,
@@ -191,6 +197,21 @@ impl Word {
         }
     }
 
+    /// A keyword that highlights as a column **type** rather than a
+    /// clause keyword (ADR-0022 Amendment 4). The one user today is
+    /// the two-word `double precision` SQL alias (ADR-0035 §3): it
+    /// is matched as keyword tokens, not an `IdentSource::Types`
+    /// `Ident`, so without this it would render keyword-coloured
+    /// while its single-word synonyms (`float`, `real`) render as
+    /// types.
+    pub const fn type_keyword(primary: &'static str) -> Self {
+        Self {
+            primary,
+            aliases: &[],
+            highlight_override: Some(HighlightClass::Type),
+        }
+    }
+
     /// Case-insensitive match against the primary or any alias.
     pub fn matches(&self, candidate: &str) -> bool {
         if candidate.eq_ignore_ascii_case(self.primary) {

src/dsl/grammar/shared.rs

@@ -7,8 +7,8 @@
 
 use crate::completion::TableColumn;
 use crate::dsl::grammar::{
-    HintMode, IdentSource, IdentValidator, Node, NumberValidator,
-    ValidationError, Word,
+    HighlightClass, HintMode, IdentSource, IdentValidator, Node,
+    NumberValidator, ValidationError, Word,
 };
 use crate::dsl::types::Type;
 use crate::dsl::walker::context::WalkContext;
@@ -50,7 +50,7 @@ pub const TYPE_SLOT: Node = Node::Ident {
     source: IdentSource::Types,
     role: "type",
     validator: Some(TYPE_VALIDATOR),
-    highlight_override: None,
+    highlight_override: Some(HighlightClass::Type),
         writes_table: false,
         writes_column: false,
         writes_user_listed_column: false,

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] = &[