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/walker/driver.rs

@@ -180,7 +180,7 @@ fn walk_node_inner(
             source: src,
             role,
             validator,
-            highlight_override: _,
+            highlight_override,
             writes_table,
             writes_column,
             writes_user_listed_column,
@@ -193,6 +193,7 @@ fn walk_node_inner(
             *src,
             role,
             *validator,
+            *highlight_override,
             *writes_table,
             *writes_column,
             *writes_user_listed_column,
@@ -336,7 +337,10 @@ fn walk_word(
         per_byte.push(ByteClass {
             start,
             end,
-            class: HighlightClass::Keyword,
+            // A keyword may opt into a non-default colour via
+            // `Word::type_keyword` (e.g. `double precision`, ADR-0022
+            // Amendment 4). Plain keywords leave it `None`.
+            class: word.highlight_override.unwrap_or(HighlightClass::Keyword),
         });
         NodeWalkResult::Matched { end, skipped: Vec::new() }
     } else {
@@ -397,6 +401,7 @@ fn walk_ident(
     src: crate::dsl::grammar::IdentSource,
     role: &'static str,
     validator: Option<crate::dsl::grammar::IdentValidator>,
+    highlight_override: Option<crate::dsl::grammar::HighlightClass>,
     writes_table: bool,
     writes_column: bool,
     writes_user_listed_column: bool,
@@ -554,7 +559,10 @@ fn walk_ident(
     per_byte.push(ByteClass {
         start,
         end,
-        class: HighlightClass::Identifier,
+        // A type slot (and any future slot that wants a non-default
+        // colour) overrides the otherwise-uniform Identifier class
+        // (issue #8 / ADR-0022 Amendment 4).
+        class: highlight_override.unwrap_or(HighlightClass::Identifier),
     });
     NodeWalkResult::Matched { end, skipped: Vec::new() }
 }