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.js

@@ -0,0 +1,59 @@
+// Canonical persistent lookup rows, shared by the storage owner and backup validator.
+// SPDX-License-Identifier: GPL-3.0-or-later
+import "./lookup-stats-identity.js";
+export const { normaliseLookupTerm, lookupStatsPrefix, lookupStatsKey } = globalThis.HDLookupStats;
+export const LOOKUP_STATS_KEY = "lookupStats";
+export const LOOKUP_STATS_ROW_PREFIX = "lookupStats:";
+
+export function emptyLookupStats() {
+  return { generation: null, revision: 0 };
+}
+
+export function assertLookupStatsDescriptor(descriptor) {
+  if (!descriptor || !Number.isSafeInteger(descriptor.revision) || descriptor.revision < 0
+      || (descriptor.generation !== null && (typeof descriptor.generation !== "string" || descriptor.generation === ""))) {
+    throw new Error("Invalid lookup statistics descriptor.");
+  }
+}
+
+function assertLookupStatsRow(row) {
+  if (!row || typeof row.term !== "string" || row.term.trim() === "" || typeof row.reading !== "string"
+      || !Number.isSafeInteger(row.lookupCount) || row.lookupCount < 0
+      || !Number.isFinite(row.firstLookedUpAt) || !Number.isFinite(row.lastLookedUpAt)
+      || row.firstLookedUpAt > row.lastLookedUpAt) {
+    throw new Error("Invalid lookup statistics row.");
+  }
+  const canonical = normaliseLookupTerm(row.term, row.reading);
+  if (row.term !== canonical.term || row.reading !== canonical.reading) {
+    throw new Error("The lookup statistics row is not canonical.");
+  }
+}
+
+export function assertLookupStatsRows(descriptor, rows) {
+  assertLookupStatsDescriptor(descriptor);
+  if (!Array.isArray(rows) || (descriptor.generation === null && rows.length !== 0)) {
+    throw new Error("Invalid lookup statistics collection.");
+  }
+  const keys = new Set();
+  for (const row of rows) {
+    assertLookupStatsRow(row);
+    const key = lookupStatsKey(descriptor, row);
+    if (keys.has(key)) throw new Error("Duplicate lookup statistics rows.");
+    keys.add(key);
+  }
+}
+
+export function incrementLookupStats(previous, term, now) {
+  if (previous !== undefined) {
+    assertLookupStatsRow(previous);
+    if (previous.term !== term.term || previous.reading !== term.reading) {
+      throw new Error("The lookup statistics row does not match its key.");
+    }
+  }
+  const row = { ...term, lookupCount: (previous?.lookupCount ?? 0) + 1,
+    firstLookedUpAt: Math.min(previous?.firstLookedUpAt ?? now, now),
+    lastLookedUpAt: Math.max(previous?.lastLookedUpAt ?? now, now) };
+  // Safe integers are the persisted JSON representation, not a product cap.
+  assertLookupStatsRow(row);
+  return row;
+}