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/anki-integration/known-word-maturity.ts

@@ -0,0 +1,102 @@
+import type { AnkiConnectConfig } from '../types/anki';
+import type { KnownWordMaturityTier } from '../types/subtitle';
+
+export const DEFAULT_MATURE_INTERVAL_THRESHOLD_DAYS = 21;
+
+// Ascending maturity; index order backs tier comparison.
+const TIER_ORDER: readonly KnownWordMaturityTier[] = ['new', 'learning', 'young', 'mature'];
+
+export interface KnownWordMaturityTierQueries {
+  mature: string;
+  young: string;
+  learning: string;
+}
+
+export interface KnownWordMaturityTierSets {
+  mature: ReadonlySet<number>;
+  young: ReadonlySet<number>;
+  learning: ReadonlySet<number>;
+}
+
+// Maturity tiers only affect how known-word highlights render, so both flags
+// must be on before tier data is fetched or served.
+export function getKnownWordMaturityEnabled(config: AnkiConnectConfig): boolean {
+  return (
+    config.knownWords?.highlightEnabled === true && config.knownWords?.maturityEnabled === true
+  );
+}
+
+export function getMatureIntervalThresholdDays(config: AnkiConnectConfig): number {
+  const threshold = config.knownWords?.matureThresholdDays;
+  if (typeof threshold === 'number' && Number.isFinite(threshold) && threshold >= 1) {
+    return Math.floor(threshold);
+  }
+  return DEFAULT_MATURE_INTERVAL_THRESHOLD_DAYS;
+}
+
+// Anki search props classify notes server-side: a note matches a tier query
+// when ANY of its cards matches, which implements most-mature-card-wins for
+// free once tiers are checked in mature > young > learning order.
+export function buildKnownWordMaturityTierQueries(
+  scopeQuery: string,
+  thresholdDays: number,
+): KnownWordMaturityTierQueries {
+  const prefix = scopeQuery.trim().length > 0 ? `${scopeQuery.trim()} ` : '';
+  return {
+    mature: `${prefix}prop:ivl>=${thresholdDays}`,
+    young: `${prefix}prop:ivl>=1 prop:ivl<${thresholdDays}`,
+    learning: `${prefix}is:learn`,
+  };
+}
+
+export async function fetchKnownWordMaturityTierSets(
+  findNotes: (query: string, options?: { maxRetries?: number }) => Promise<unknown>,
+  scopeQueries: string[],
+  thresholdDays: number,
+): Promise<{ mature: Set<number>; young: Set<number>; learning: Set<number> }> {
+  const sets = {
+    mature: new Set<number>(),
+    young: new Set<number>(),
+    learning: new Set<number>(),
+  };
+  for (const scopeQuery of scopeQueries) {
+    const queries = buildKnownWordMaturityTierQueries(scopeQuery, thresholdDays);
+    for (const tier of ['mature', 'young', 'learning'] as const) {
+      const noteIds = (await findNotes(queries[tier], { maxRetries: 0 })) as number[];
+      if (!Array.isArray(noteIds)) {
+        continue;
+      }
+      for (const noteId of noteIds) {
+        if (Number.isInteger(noteId) && noteId > 0) {
+          sets[tier].add(noteId);
+        }
+      }
+    }
+  }
+  return sets;
+}
+
+export function classifyKnownWordNoteTier(
+  noteId: number,
+  sets: KnownWordMaturityTierSets,
+): KnownWordMaturityTier {
+  if (sets.mature.has(noteId)) return 'mature';
+  if (sets.young.has(noteId)) return 'young';
+  if (sets.learning.has(noteId)) return 'learning';
+  return 'new';
+}
+
+export function maxKnownWordMaturityTier(
+  a: KnownWordMaturityTier | null | undefined,
+  b: KnownWordMaturityTier | null | undefined,
+): KnownWordMaturityTier | null {
+  if (!a) return b ?? null;
+  if (!b) return a;
+  return TIER_ORDER.indexOf(a) >= TIER_ORDER.indexOf(b) ? a : b;
+}
+
+export function sanitizeKnownWordMaturityTier(value: unknown): KnownWordMaturityTier | null {
+  return typeof value === 'string' && TIER_ORDER.includes(value as KnownWordMaturityTier)
+    ? (value as KnownWordMaturityTier)
+    : null;
+}