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/response-limits.js

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+const MAX_LOOKUP_RESPONSE_BYTES = 32 * 1024 * 1024;
+const MAX_MEDIA_RESPONSE_BYTES = 6 * 1024 * 1024;
+// Browser adaptation of the source's 1 MiB control frame, only for reader
+// options messages. Dictionary state, custom source, and archives are separate.
+const MAX_OPTIONS_FRAME_BYTES = 1024 * 1024;
+const BOUNDED_REQUESTS = new Set([
+  "hd_frequencies",
+  "hd_lookup",
+  "hd_lookup_dictionary",
+  "hd_kanji",
+  "hd_media",
+  "hd_capture_asset",
+  "hd_options_write",
+]);
+const responseFrameEncoder = new TextEncoder();
+
+export function isBoundedRequest(type) {
+  return BOUNDED_REQUESTS.has(type);
+}
+
+export function responseLimitError(type) {
+  if (type === "hd_options_write" || type === "hd_options_write_result") {
+    return "reader options message exceeds the 1 MiB serialized limit";
+  }
+  return ["hd_media", "hd_media_result", "hd_capture_asset", "hd_capture_asset_result"].includes(type)
+    ? "media response exceeds the 6 MiB serialized limit"
+    : "lookup response exceeds the 32 MiB serialized limit";
+}
+
+export function validResponseRequestId(value) {
+  return value === null || typeof value === "string" || Number.isFinite(value);
+}
+
+export function responseFits(reply, nativeJsonLength = 0) {
+  if (reply.type === "hd_options_write" || reply.type === "hd_options_write_result") {
+    const json = JSON.stringify(reply);
+    // UTF-8 cannot be shorter than JSON's UTF-16 code-unit count.
+    if (json.length > MAX_OPTIONS_FRAME_BYTES) return false;
+    return json.length * 3 <= MAX_OPTIONS_FRAME_BYTES
+      || responseFrameEncoder.encode(json).byteLength <= MAX_OPTIONS_FRAME_BYTES;
+  }
+  if (reply.type === "hd_media_result" || reply.type === "hd_capture_asset_result") {
+    // The producer constructs dataUrl only from fixed ASCII MIME strings and
+    // base64. Count that payload exactly without serializing/copying it again.
+    const field = reply.type === "hd_media_result" ? "dataUrl" : "data";
+    const dataLength = typeof reply[field] === "string" ? reply[field].length : 0;
+    const frame = JSON.stringify(dataLength ? { ...reply, [field]: "" } : reply);
+    return frame.length * 3 + dataLength <= MAX_MEDIA_RESPONSE_BYTES
+      || responseFrameEncoder.encode(frame).byteLength + dataLength <= MAX_MEDIA_RESPONSE_BYTES;
+  }
+  // Extra empty endpoint fields only enlarge this conservative envelope; near
+  // the boundary we always measure the actual reply instead.
+  const envelope = nativeJsonLength === 0 ? reply : { ...reply, results: [], kanji: null };
+  const envelopeJson = JSON.stringify(envelope);
+  // JSON escaping needs at most six bytes per UTF-16 code unit, including
+  // lone surrogates. Ordinary native replies need no second full traversal.
+  if ((nativeJsonLength + envelopeJson.length) * 6 <= MAX_LOOKUP_RESPONSE_BYTES) return true;
+  const json = nativeJsonLength === 0 ? envelopeJson : JSON.stringify(reply);
+  return responseFrameEncoder.encode(json).byteLength <= MAX_LOOKUP_RESPONSE_BYTES;
+}
+
+export function boundResponseFailure(reply) {
+  if (!isBoundedRequest(reply.type.replace(/_result$/u, ""))) return reply;
+  if (!validResponseRequestId(reply.requestId)) reply.requestId = null;
+  if (!responseFits(reply)) {
+    const limitError = responseLimitError(reply.type);
+    const unchangedError = reply.error === limitError;
+    reply.error = limitError;
+    // Replacing an identical error cannot make the same oversized frame fit.
+    if (unchangedError || !responseFits(reply)) reply.requestId = null;
+  }
+  return reply;
+}