feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

vendor/hachidori/extension/audio-cache.js

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Shared retention policy for candidate metadata and offscreen-owned media.
+// A value too large to retain is still usable by its caller, uncached.
+export function createAudioCache({ maxEntries, maxBytes, ttlMs, onEvict, now = () => performance.now() }) {
+  const entries = new Map();
+  let retainedBytes = 0;
+
+  function remove(key) {
+    const entry = entries.get(key);
+    if (!entry) return;
+    entries.delete(key);
+    retainedBytes -= entry.bytes;
+    onEvict?.(entry.value);
+  }
+
+  return {
+    delete: remove,
+    get(key) {
+      const entry = entries.get(key);
+      if (!entry) return undefined;
+      if (entry.expiresAt <= now()) { remove(key); return undefined; }
+      entries.delete(key);
+      entries.set(key, entry);
+      return entry.value;
+    },
+    set(key, value, bytes) {
+      remove(key);
+      if (bytes > maxBytes) return false;
+      entries.set(key, { value, bytes, expiresAt: now() + ttlMs });
+      retainedBytes += bytes;
+      while (entries.size > maxEntries || retainedBytes > maxBytes) remove(entries.keys().next().value);
+      return true;
+    },
+    clear() { for (const key of entries.keys()) remove(key); },
+  };
+}