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/capture-audio-worklet.js

@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+class HachidoriCaptureProcessor extends AudioWorkletProcessor {
+  constructor() {
+    super();
+    this.batch = new Float32Array(2048);
+    this.length = 0;
+    this.startFrame = 0;
+    this.nextFrame = null;
+  }
+
+  inputInterrupted(frameCount) {
+    if (this.nextFrame !== null && (frameCount === 0 || currentFrame !== this.nextFrame)) {
+      const expectedFrame = this.nextFrame;
+      this.length = 0;
+      this.nextFrame = frameCount ? currentFrame + frameCount : null;
+      this.port.postMessage({
+        discontinuity: {
+          expectedFrame,
+          actualFrame: frameCount ? currentFrame : null,
+        },
+      });
+      return true;
+    }
+    this.nextFrame = frameCount ? currentFrame + frameCount : null;
+    return false;
+  }
+
+  // Empty input can precede the first active quantum. Returning false allows
+  // Chrome to permanently retire this processor before the source is ready.
+  process(inputs) { // NOSONAR -- S3516: the AudioWorklet lifetime contract requires true.
+    const channels = inputs[0];
+    const frameCount = channels?.[0]?.length ?? 0;
+    this.inputInterrupted(frameCount);
+    if (frameCount === 0) return true;
+    for (let frame = 0; frame < frameCount; frame += 1) {
+      if (this.length === 0) this.startFrame = currentFrame + frame;
+      let sample = 0;
+      for (const channel of channels) sample += channel[frame] ?? 0;
+      this.batch[this.length] = sample / channels.length;
+      this.length += 1;
+      if (this.length === this.batch.length) {
+        const samples = this.batch;
+        this.port.postMessage({ startFrame: this.startFrame, samples: samples.buffer }, [samples.buffer]);
+        this.batch = new Float32Array(2048);
+        this.length = 0;
+      }
+    }
+    return true;
+  }
+}
+
+registerProcessor("hachidori-capture-audio", HachidoriCaptureProcessor);