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/anki-offscreen.js

@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+import { extensionApi } from "./browser-api.js";
+import { decodeBase64 } from "./base64.js";
+import { buildAnkiResourceFields } from "./anki-resources.js";
+import { exportAnkiAudio } from "./anki-audio.js";
+import { MINING_CAPABILITIES } from "./overlay-mode.js";
+
+// Resolve and parse the complete scoped note set away from the background and
+// engine request threads; only compact index rows cross back to the commit.
+async function refreshAnkiIndex(window, source) {
+  const worker = new window.Worker(new URL("./anki-index-worker.js", import.meta.url), { type: "module" });
+  try {
+    return await new Promise((resolve, reject) => {
+      worker.onmessage = ({ data }) => data.error ? reject(new Error(data.error)) : resolve({ rows: data.rows });
+      worker.onerror = event => reject(new Error(event.message || "Anki index refresh worker failed."));
+      worker.postMessage(source);
+    });
+  } finally {
+    worker.terminate();
+  }
+}
+
+async function recordSpeechAudio(...args) {
+  if (!MINING_CAPABILITIES.browserSpeech) {
+    throw new Error("Browser text-to-speech recording is unavailable in Firefox.");
+  }
+  const capture = await import("./capture-host.js");
+  return capture.recordSpeechAudio(...args);
+}
+
+function clientSpeechPlan(source, term) {
+  return {
+    sourceId: source.id,
+    sourceKey: JSON.stringify(source),
+    expression: term.expression,
+    reading: term.reading,
+  };
+}
+
+function sameClientSpeech(plan, source, term) {
+  const expected = clientSpeechPlan(source, term);
+  return plan && Object.entries(expected).every(([key, value]) => plan[key] === value);
+}
+
+function decodeClientSpeech(window, data) {
+  return decodeBase64(data, { atob: window.atob.bind(window) });
+}
+
+function linkedSpeechRecorder(window, message) {
+  if (message.clientSpeechProbe !== true && !message.clientSpeech) return recordSpeechAudio;
+  return async (source, term, signal, { record = true } = {}) => {
+    signal.throwIfAborted();
+    const plan = clientSpeechPlan(source, term);
+    if (!record || message.clientSpeechProbe === true) {
+      return { recordingRequired: true, clientSpeech: plan };
+    }
+    const supplied = message.clientSpeech;
+    if (!sameClientSpeech(supplied, source, term)) {
+      throw new Error("The linked browser did not supply the requested browser speech.");
+    }
+    return {
+      data: decodeClientSpeech(window, supplied.data),
+      filename: supplied.filename,
+      candidate: { name: "Linked browser speech", text: term.reading || term.expression, voice: "", index: 0 },
+    };
+  };
+}
+
+export function createAnkiOffscreenService(window, getAudioRepository, captureSpeech = recordSpeechAudio) {
+  return async message => {
+    if (message.type === "hd_anki_index_refresh") return refreshAnkiIndex(window, message.source);
+    if (message.type === "hd_anki_audio") {
+      return exportAnkiAudio(window, await getAudioRepository(), message, window.AbortSignal.timeout(30_000), {
+        recordSpeechAudio: message.clientSpeechProbe === true || message.clientSpeech
+          ? linkedSpeechRecorder(window, message)
+          : captureSpeech,
+      });
+    }
+    if (message.type !== "hd_anki_fields") throw new Error("Unknown Anki rendering request.");
+    return buildAnkiResourceFields(message.request, message.templates, {
+      document: window.document, dictionaryPaths: message.dictionaryPaths, audio: message.audio,
+      styles: async () => {
+        const reply = await extensionApi.runtime.sendMessage({ target: "hoshidicts-offscreen", type: "hd_styles", requestId: message.requestId });
+        if (!reply.ok || reply.generation !== message.request.generation) throw new Error(reply.error || "Dictionary styles changed during Anki preparation.");
+        return reply.styles;
+      },
+    });
+  };
+}