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/opfs-capability-worker.js

@@ -0,0 +1,62 @@
+/*
+ * Verifies the synchronous OPFS primitives required by WasmFS before the
+ * offscreen document commits to the pthread runtime.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+function describe(error) {
+  return error instanceof Error ? error.message || String(error) : String(error);
+}
+
+// A dedicated worker receives only from its creator over its implicit
+// MessagePort. MessageEvent.origin is always empty, so there is no origin value
+// to validate; the channel check below validates the expected protocol.
+globalThis.onmessage = async (event) => { // NOSONAR
+  if (event.data?.channel !== "opfs-capability-probe") return;
+
+  const suffix = `${Date.now()}-${globalThis.crypto.randomUUID()}`;
+  const first = `.hdw-opfs-probe-${suffix}`;
+  const second = `${first}-moved`;
+  let root = null;
+  let access = null;
+  try {
+    root = await navigator.storage.getDirectory();
+    const handle = await root.getFileHandle(first, { create: true });
+    if (typeof handle.createSyncAccessHandle !== "function" || typeof handle.move !== "function") {
+      throw new Error("OPFS synchronous access or file move is unavailable");
+    }
+    access = await handle.createSyncAccessHandle();
+    access.write(new Uint8Array([0x48, 0x44, 0x57]), { at: 0 });
+    access.flush();
+    access.close();
+    access = null;
+
+    await handle.move(root, second);
+    const moved = await root.getFileHandle(second);
+    access = await moved.createSyncAccessHandle();
+    if (access.getSize() !== 3) {
+      throw new Error("an OPFS file did not survive move intact");
+    }
+    access.close();
+    access = null;
+    await root.removeEntry(second);
+    globalThis.postMessage({ channel: "opfs-capability-result", ok: true });
+  } catch (error) {
+    try {
+      access?.close();
+    } catch {
+      // The failed operation may already have closed it.
+    }
+    if (root !== null) {
+      for (const name of [first, second]) {
+        try {
+          await root.removeEntry(name);
+        } catch {
+          // A probe path that was never created or already moved is absent.
+        }
+      }
+    }
+    globalThis.postMessage({ channel: "opfs-capability-result", ok: false, error: describe(error) });
+  }
+};