feat(overlay): Anki maturity-based known-word highlighting

Color known-word subtitle highlights by Anki card maturity (new,
learning, young, mature) like asbplayer (#171). Notes are classified
server-side with Anki search filters (prop:ivl, is:learn) during the
known-word cache refresh, so no per-card data is fetched. A word's tier
is its most mature matching card/note, with the same reading-aware
matching as boolean known-word lookups.

- known-word cache v4 state persists per-note tiers; lifecycle key only
  gains the maturity field while enabled so existing caches survive
- ankiConnect.knownWords.maturityEnabled + matureThresholdDays (21)
- subtitleStyle.knownWordMaturityColors with catppuccin defaults
- word-maturity-<tier> class rides on word-known so hover/selection
  rules keep applying; falls back to knownWordColor without tier data
- runtime toggle subtitle.annotation.knownWords.maturityEnabled

src/config/definitions/options-integrations.ts

@@ -295,6 +295,20 @@ export function buildIntegrationConfigOptionRegistry(
       defaultValue: defaultConfig.ankiConnect.knownWords.highlightEnabled,
       description: 'Enable fast local highlighting for words already known in Anki.',
     },
+    {
+      path: 'ankiConnect.knownWords.maturityEnabled',
+      kind: 'boolean',
+      defaultValue: defaultConfig.ankiConnect.knownWords.maturityEnabled,
+      description:
+        'Color known-word highlights by Anki card maturity (new, learning, young, mature) instead of a single color. Requires known-word highlighting.',
+    },
+    {
+      path: 'ankiConnect.knownWords.matureThresholdDays',
+      kind: 'number',
+      defaultValue: defaultConfig.ankiConnect.knownWords.matureThresholdDays,
+      description:
+        'Card interval in days at which a known word counts as mature (Anki convention: 21).',
+    },
     {
       path: 'ankiConnect.knownWords.refreshMinutes',
       kind: 'number',