refactor(live-atc): extract encodeWav to shared/utils, add tests

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

shared/utils/wavEncoder.ts

@@ -0,0 +1,28 @@
+export const encodeWav = (pcm: Float32Array, sampleRate: number): Blob => {
+  const length = pcm.length
+  const buffer = new ArrayBuffer(44 + length * 2)
+  const view = new DataView(buffer)
+  const writeStr = (off: number, s: string) => {
+    for (let i = 0; i < s.length; i++) view.setUint8(off + i, s.charCodeAt(i))
+  }
+  writeStr(0, 'RIFF')
+  view.setUint32(4, 36 + length * 2, true)
+  writeStr(8, 'WAVE')
+  writeStr(12, 'fmt ')
+  view.setUint32(16, 16, true)
+  view.setUint16(20, 1, true)
+  view.setUint16(22, 1, true)
+  view.setUint32(24, sampleRate, true)
+  view.setUint32(28, sampleRate * 2, true)
+  view.setUint16(32, 2, true)
+  view.setUint16(34, 16, true)
+  writeStr(36, 'data')
+  view.setUint32(40, length * 2, true)
+  let off = 44
+  for (let i = 0; i < length; i++) {
+    const s = Math.max(-1, Math.min(1, pcm[i]))
+    view.setInt16(off, s < 0 ? s * 0x8000 : s * 0x7FFF, true)
+    off += 2
+  }
+  return new Blob([buffer], { type: 'audio/wav' })
+}