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/lookup-stats-identity.js

@@ -0,0 +1,22 @@
+// Shared by native modules and manifest-loaded content scripts.
+// SPDX-License-Identifier: GPL-3.0-or-later
+(function () {
+  "use strict";
+
+  function normaliseLookupTerm(term, reading = "") {
+    if (typeof term !== "string" || typeof reading !== "string" || term.trim() === "") {
+      throw new TypeError("Lookup statistics require a nonempty term and a reading string.");
+    }
+    return { term: term.trim().normalize("NFC"), reading: reading.trim().normalize("NFC") };
+  }
+
+  function lookupStatsPrefix(descriptor) {
+    return `lookupStats:${JSON.stringify(descriptor.generation)}:`;
+  }
+
+  function lookupStatsKey(descriptor, { term, reading }) {
+    return lookupStatsPrefix(descriptor) + JSON.stringify([term, reading]);
+  }
+
+  globalThis.HDLookupStats = { normaliseLookupTerm, lookupStatsPrefix, lookupStatsKey };
+}());