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/kernel/kernel.dart

@@ -45,6 +45,7 @@ export 'src/focus.dart';
 export 'src/i18n/catalog_loader.dart';
 export 'src/i18n/fallback_chain.dart';
 export 'src/i18n/i18n.dart';
+export 'src/i18n/tier0_namespaces.dart';
 export 'src/net.dart';
 export 'src/notify.dart';
 export 'src/os.dart';
@@ -63,6 +64,7 @@ export 'src/panels/layout_preset.dart';
 export 'src/panels/registry.dart';
 export 'src/panels/slot_id.dart';
 export 'src/panels/view_pane_snapshot.dart';
+export 'src/theme/bundled_themes.dart';
 export 'src/theme/contrast.dart';
 export 'src/theme/controller.dart';
 export 'src/theme/loader.dart';