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/transcoder.go

@@ -27,6 +27,11 @@ type TranscodeOpts struct {
 	SourceHeight int    // probed source height — used to derive a sane H.264 level
 	StartSeconds float64
 	FFmpegPath   string
+	// VideoTag forces the output stream's codec tag on a copy remux. HEVC muxed
+	// into MP4 must carry the `hvc1` tag (not the default `hev1`) or Safari /
+	// Apple devices refuse to decode it. Empty = leave ffmpeg's default. Only
+	// applied on copy actions (passthrough/remux); a real re-encode sets its own.
+	VideoTag string
 }
 
 // Transcoder wraps a long-running ffmpeg child process whose stdout streams
@@ -222,8 +227,16 @@ func buildFFmpegArgs(filePath string, opts TranscodeOpts) []string {
 	switch opts.Action {
 	case ActionPassthrough, ActionRemux:
 		args = append(args, "-c:v", "copy", "-c:a", "copy")
+		// HEVC → MP4 needs the hvc1 tag for Apple/Safari (hueco #3 / 3c).
+		if opts.VideoTag != "" {
+			args = append(args, "-tag:v", opts.VideoTag)
+		}
 	case ActionRemuxAudio:
-		args = append(args, "-c:v", "copy", "-c:a", "aac", "-b:a", coalesce(opts.AudioBitrate, "192k"))
+		args = append(args, "-c:v", "copy")
+		if opts.VideoTag != "" {
+			args = append(args, "-tag:v", opts.VideoTag) // HEVC → hvc1 for Apple
+		}
+		args = append(args, "-c:a", "aac", "-b:a", coalesce(opts.AudioBitrate, "192k"))
 	case ActionTranscodeVideo:
 		videoCodec := opts.HWAccel.FFmpegVideoCodec("h264")
 		args = append(args, "-c:v", videoCodec)