feat(web-test): per-caption voice + speechRate for multi-voice narration

- addNarration: use cap.voice override per caption (fallback to global)
- showCaption/showImage/showTitleSlide: pass opts.voice to caption entry
- showCaption: record caption when text is empty but speech is explicit
- startRecording: add speechRate option (default 70ms/char, 85 for ElevenLabs)
- run.mjs: increase exec timeout to 30min for long recordings
- docs: update recording.md and web-test-recording-guide.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/web-test/scripts/run.mjs

@@ -230,7 +230,7 @@ async function cmdExec(fileOrDash, flags = {}) {
   const result = await new Promise((resolve, reject) => {
     const req = http.request({
       hostname: '127.0.0.1', port: sess.port, path: '/exec',
-      method: 'POST', timeout: 10 * 60 * 1000, headers,
+      method: 'POST', timeout: 30 * 60 * 1000, headers,
     }, res => {
       let data = '';
       res.on('data', chunk => data += chunk);