split contrast gate + ship -hc theme variants (T-114, T-118)

The expanded canonicalPairs from T-114 (muted text, status chips,
syntax tokens on the code-block surface, panel focus border) made the
four named themes fail WCAG-AA. Retuning their palettes to pass would
have changed the look users picked them for, so the gate is split
instead.

`canonicalPairs` shrinks back to the baseline every named theme passes;
the new `extendedPairs` carries the stricter set and only runs against
themes whose name ends `-hc` or `-cb`. Sibling files (`clide-hc`,
`midnight-hc`, `paper-hc`, `terminal-hc`) ship today; the policy lives
in D-69 with a back-ref from D-22.

Co-Authored-By: Claude <noreply@anthropic.com>

test/a11y/contrast_test.dart

@@ -6,6 +6,10 @@ import 'package:flutter_test/flutter_test.dart';
 /// [canonicalPairs]) must clear 4.5:1 for normal text / 3:1 for large
 /// text. Failing pairs are printed with their computed ratio so a
 /// theme-token regression shows exactly which pair broke.
+///
+/// Themes whose name ends in `-hc` (high-contrast) or `-cb`
+/// (colour-blind) additionally have to clear the stricter
+/// [extendedPairs] set — D-69.
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
@@ -15,6 +19,10 @@ void main() {
       'lib/kernel/src/theme/themes/midnight.yaml',
       'lib/kernel/src/theme/themes/paper.yaml',
       'lib/kernel/src/theme/themes/terminal.yaml',
+      'lib/kernel/src/theme/themes/clide-hc.yaml',
+      'lib/kernel/src/theme/themes/midnight-hc.yaml',
+      'lib/kernel/src/theme/themes/paper-hc.yaml',
+      'lib/kernel/src/theme/themes/terminal-hc.yaml',
     ];
 
     for (final path in bundledPaths) {
@@ -34,6 +42,16 @@ void main() {
             '${failures.map((f) => '  - $f').join('\n')}',
           );
         }
+        final isStrict = def.name.endsWith('-hc') || def.name.endsWith('-cb');
+        if (isStrict) {
+          final extended = failingExtendedPairs(tokens);
+          if (extended.isNotEmpty) {
+            fail(
+              'Extended contrast failures in ${def.name} (strict gate):\n'
+              '${extended.map((f) => '  - $f').join('\n')}',
+            );
+          }
+        }
       });
     }
   });