feat(theme): canonical theme/namespace lists + ship Summer Night (T-371, T-478)

The bundled-theme list and the Tier-0 i18n namespace list were each
hand-maintained in 3+ places that had drifted: the testmode harness loaded
8 themes (catppuccin silently unvalidated) while the app and contrast gate
loaded 10, and the i18n gate checked 4 of the shipped namespaces. Export
one canonical const each — kBundledThemePaths and kTier0Namespaces — and
have the app, the testmode harness, and the a11y gates iterate them.

Drift-proof meta-assertions:
- contrast gate fails if any theme YAML on disk is absent from
  kBundledThemePaths (so a theme can't sit unvalidated).
- i18n gate derives its subjects from assets/i18n/en_us/ at test time and
  asserts en/nl key parity for every shipped catalog (26, was 4), plus that
  every kTier0Namespaces entry has a shipped catalog.

Surfaced summer-night.yaml: a legacy-ported palette on disk, unbundled and
never contrast-checked. Per the maintainer's call, ship it: flesh the
palette to full token + syntax coverage (honouring the v1.2 colours, clide
derivation pattern for the new keys) and add a structurally identical
summer-night-hc sibling that clears the strict extended gate. Both pass.

No licenses.yaml change: summer-night is clide's own (ported from legacy
clide v1.2 under legacy/, MIT); the only third-party palette, Catppuccin,
is already acknowledged; no fonts added.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/test_app.dart

@@ -247,17 +247,7 @@ class _ClideTestAppState extends State<ClideTestApp> {
     // Theme loading
     try {
       const loader = ThemeLoader();
-      const paths = [
-        'lib/kernel/src/theme/themes/clide.yaml',
-        '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 p in paths) {
+      for (final p in kBundledThemePaths) {
         final name = p.split('/').last.replaceAll('.yaml', '');
         try {
           final theme = await loader.fromAsset(rootBundle, p);
@@ -277,7 +267,7 @@ class _ClideTestAppState extends State<ClideTestApp> {
       final themes = <ThemeDefinition>[];
       try {
         const loader = ThemeLoader();
-        themes.add(await loader.fromAsset(rootBundle, 'lib/kernel/src/theme/themes/clide.yaml'));
+        themes.add(await loader.fromAsset(rootBundle, kBundledThemePaths.first));
       } catch (_) {}
 
       final services = await KernelServices.boot(