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/shared/ipc/validators.ts

@@ -54,6 +54,7 @@ const SESSION_ACTION_IDS: SessionActionId[] = [
 const RUNTIME_OPTION_IDS: RuntimeOptionId[] = [
   'anki.autoUpdateNewCards',
   'subtitle.annotation.knownWords.highlightEnabled',
+  'subtitle.annotation.knownWords.maturityEnabled',
   'subtitle.annotation.nPlusOne',
   'subtitle.annotation.jlpt',
   'subtitle.annotation.frequency',