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/texthooker-protocol.js

@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+const IGNORED_PLAIN_MESSAGES = new Set(["True", "False"]);
+export const MAX_TEXTHOOKER_FRAME_LENGTH = 64 * 1024;
+export const MAX_TEXTHOOKER_TEXT_LENGTH = 4096;
+
+export function parsePlainTexthookerMessage(value) {
+  if (typeof value !== "string" || value.length > MAX_TEXTHOOKER_FRAME_LENGTH
+      || !value.trim() || value.length > MAX_TEXTHOOKER_TEXT_LENGTH
+      || IGNORED_PLAIN_MESSAGES.has(value.trim())) return null;
+  try {
+    JSON.parse(value);
+    return null;
+  } catch {
+    return { type: "line", text: value };
+  }
+}
+
+export function parseGsmTexthookerMessage(value) {
+  if (typeof value === "string" && value.length > MAX_TEXTHOOKER_FRAME_LENGTH) return null;
+  let payload;
+  try {
+    payload = typeof value === "string" ? JSON.parse(value) : value;
+  } catch {
+    return null;
+  }
+  if (!payload || typeof payload !== "object" || Array.isArray(payload)) return null;
+  if (["reset", "reset_checkboxes", "session_reset"].includes(payload.event)) return { type: "reset" };
+  const data = payload.data;
+  if (payload.event !== "text_received" || !data || typeof data !== "object" || Array.isArray(data)
+      || typeof data.id !== "string" || !data.id
+      || typeof data.session_id !== "string" || !data.session_id
+      || data.history === true) return null;
+  let text = payload.sentence;
+  if (typeof text !== "string") text = typeof data.text === "string" ? data.text : "";
+  if (!text.trim() || text.length > MAX_TEXTHOOKER_TEXT_LENGTH) return null;
+  return {
+    type: "line",
+    id: data.id,
+    sessionId: data.session_id,
+    text,
+  };
+}
+
+export function parseTexthookerMessage(format, value) {
+  if (format === "plain") return parsePlainTexthookerMessage(value);
+  if (format === "gsm") return parseGsmTexthookerMessage(value);
+  throw new Error("unsupported texthooker format");
+}