feat(stream): device-aware remux (HEVC/AV1 + non-aac audio) + TTFF timers

Hueco #3 / 3c (CLI). NewRemuxSource now copies the video for any
browser-decodable codec: h264, or HEVC/AV1 when the web says the device
decodes them (caps). HEVC is muxed with -tag:v hvc1 (Apple requirement),
and non-aac audio (ac3/eac3/dts) is transcoded to aac while the video is
still copied (ActionRemuxAudio) — this covers the very common h264+ac3 mkv.

Startup instrumentation for time-to-first-frame diagnosis:
- remux branch logs [probe=.. spawn=..]
- transcodeSource logs 'first fMP4 bytes after ..' (ffmpeg → first output)
- serveGrowing logs reads that block >250ms (client seeking ahead of the
  live edge) + the first read's offset vs produced/estimated size.

Verified: caps gate (hls without caps, remux with), hvc1 retag (ffprobe of
the /stream output = hevc/hvc1), HEVC playback confirmed on a real iPhone
Safari over Tailscale. LAN timeline: probe 16ms, spawn 1ms, first byte
201ms, no serveGrowing blocks.

internal/engine/stream_server.go

@@ -863,6 +863,7 @@ func (ss *StreamServer) serveGrowing(w http.ResponseWriter, r *http.Request, src
 
 	buf := make([]byte, 256*1024)
 	off := start
+	firstRead := true
 	for {
 		if explicitEnd >= 0 && off > explicitEnd {
 			return
@@ -870,7 +871,19 @@ func (ss *StreamServer) serveGrowing(w http.ResponseWriter, r *http.Request, src
 		if r.Context().Err() != nil {
 			return // client disconnected / request cancelled
 		}
+		readStart := time.Now()
 		n, err := src.ReadAt(buf, off)
+		// TTFF diagnosis: a read that blocks means the client asked for bytes the
+		// remux hasn't produced yet (a seek ahead of the live edge, or the very
+		// first read before ffmpeg's init lands). Log it so a slow start is
+		// attributable to "waiting on ffmpeg" vs network/decoder.
+		if waited := time.Since(readStart); waited > 250*time.Millisecond {
+			log.Printf("[stream] serveGrowing read off=%d blocked %v (produced=%d est=%d)",
+				off, waited.Round(time.Millisecond), src.Size(), src.EstimatedSize())
+		} else if firstRead {
+			log.Printf("[stream] serveGrowing start off=%d (produced=%d est=%d)", start, src.Size(), src.EstimatedSize())
+		}
+		firstRead = false
 		if n > 0 {
 			toWrite := n
 			if explicitEnd >= 0 {