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/runtime-options.ts

@@ -35,6 +35,22 @@ export function buildRuntimeOptionRegistry(
         },
       }),
     },
+    {
+      id: 'subtitle.annotation.knownWords.maturityEnabled',
+      path: 'ankiConnect.knownWords.maturityEnabled',
+      label: 'Known Word Maturity Colors',
+      scope: 'subtitle',
+      valueType: 'boolean',
+      allowedValues: [true, false],
+      defaultValue: defaultConfig.ankiConnect.knownWords.maturityEnabled,
+      requiresRestart: false,
+      formatValueForOsd: (value) => (value === true ? 'On' : 'Off'),
+      toAnkiPatch: (value) => ({
+        knownWords: {
+          maturityEnabled: value === true,
+        },
+      }),
+    },
     {
       id: 'subtitle.annotation.nPlusOne',
       path: 'ankiConnect.nPlusOne.enabled',