feat(anki): add waveform-guided timing controls

- Add speech-weighted waveform analysis and playback playhead
- Support dragging, sliding, and keyboard nudging for clip timing

src/core/services/media-timing-waveform.test.ts

@@ -0,0 +1,74 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  buildSpeechWaveformArgs,
+  computeWaveformPeaks,
+  generateSpeechWaveform,
+} from './media-timing-waveform';
+
+function pcm(samples: number[]): Buffer {
+  const result = Buffer.alloc(samples.length * 2);
+  samples.forEach((sample, index) => result.writeInt16LE(sample, index * 2));
+  return result;
+}
+
+test('speech waveform maps the selected FFmpeg stream and visible range', () => {
+  const args = buildSpeechWaveformArgs(
+    {
+      mediaPath: '/video/show.mkv',
+      startTime: 8,
+      endTime: 15,
+      audioStreamIndex: 3,
+    },
+    'center',
+  );
+
+  assert.deepEqual(args.slice(args.indexOf('-ss'), args.indexOf('-t') + 2), [
+    '-ss',
+    '8',
+    '-i',
+    '/video/show.mkv',
+    '-t',
+    '7',
+  ]);
+  assert.deepEqual(args.slice(args.indexOf('-map'), args.indexOf('-map') + 2), ['-map', '0:3']);
+  assert.match(args[args.indexOf('-af') + 1] ?? '', /c0=FC/);
+});
+
+test('waveform peaks are normalized without flattening quieter sections', () => {
+  const peaks = computeWaveformPeaks(pcm([0, 1_000, -2_000, 4_000, -8_000, 16_000]), 3);
+
+  assert.equal(peaks.length, 3);
+  assert.ok((peaks[0] ?? 0) > 0);
+  assert.ok((peaks[0] ?? 0) < (peaks[1] ?? 0));
+  assert.ok((peaks[1] ?? 0) < (peaks[2] ?? 0));
+  assert.equal(peaks[2], 1);
+});
+
+test('speech waveform uses a mono downmix when the source has no center activity', async () => {
+  const calls: string[][] = [];
+  const peaks = await generateSpeechWaveform(
+    { mediaPath: '/video/show.mkv', startTime: 0, endTime: 2 },
+    async (args) => {
+      calls.push(args);
+      return calls.length === 1 ? pcm([0, 0, 0, 0]) : pcm([0, 4_000, -8_000, 16_000]);
+    },
+  );
+
+  assert.equal(calls.length, 2);
+  assert.match(calls[1]?.[calls[1].indexOf('-af') + 1] ?? '', /channel_layouts=mono/);
+  assert.equal(Math.max(...peaks), 1);
+});
+
+test('speech waveform keeps an active center channel without doing a second decode', async () => {
+  let calls = 0;
+  await generateSpeechWaveform(
+    { mediaPath: '/video/show.mkv', startTime: 0, endTime: 2 },
+    async () => {
+      calls += 1;
+      return pcm([0, 4_000, -8_000, 16_000]);
+    },
+  );
+
+  assert.equal(calls, 1);
+});