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/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) {