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/src/i18n/tier0_namespaces.dart

@@ -0,0 +1,25 @@
+/// Canonical list of Tier-0 i18n namespaces preloaded at boot — the single
+/// source of truth (T-371).
+///
+/// `lib/main.dart` preloads exactly these (framework chrome under `core`, plus
+/// the catalogs of the built-ins that activate at Tier 0); the i18n coverage
+/// gate validates every one of them. The other ~17 catalogs that ship belong
+/// to built-ins that activate lazily in later tiers — their catalogs load on
+/// activation, and the gate validates them through the assets-dir sweep rather
+/// than this preload list.
+library;
+
+/// Namespaces whose catalogs are loaded at boot, before any extension that
+/// owns them has activated. `core` holds framework chrome that lives outside
+/// any extension (lib/widgets, lib/kernel, shared reader chrome — T-469).
+const List<String> kTier0Namespaces = [
+  'core',
+  'builtin.default-layout',
+  'builtin.welcome',
+  'builtin.ipc-status',
+  'builtin.theme-picker',
+  'builtin.terminal',
+  'builtin.files',
+  'builtin.claude',
+  'builtin.editor',
+];