feat(stream): progressive fMP4 remux source for /stream (hueco #3 / 3b-i)

Agent side of 3b: serve a growing ffmpeg `-c copy` remux (mkv h264/aac →
fragmented MP4) over /stream with no video re-encode. Dormant until the web
sends PlayMethod="remux" (3b-ii), so this commit changes no live behavior.

- GrowingSource interface + transcodeSource already satisfies it; estimate is
  the source file size for copy actions (≈ remux output) vs bitrate×duration
  for real transcodes.
- NewRemuxSource: ffmpeg -c copy → growing fMP4 temp, returned as GrowingSource.
- StreamServer.SetGrowingFile + serveGrowing: manual Range responder for a
  growing source (http.ServeContent needs a fixed size). 206 with an estimated
  total in Content-Range; chunked body while not final (never promise bytes a
  running remux might not produce); exact Content-Length once final. Blocks via
  ReadAt for not-yet-produced bytes; forward seek waits, backward seek instant.
- daemon OnStreamSession: PlayMethod=="remux" → NewRemuxSource + SetGrowingFile
  + MarkSessionReady (after the ffmpeg check; copy still needs ffmpeg).
- Tests: parseByteRange + serveGrowing (full/offset/bounded/estimate/HEAD/416).

internal/engine/stream_source.go

@@ -125,7 +125,20 @@ func newTranscodeSource(
 		return nil, err
 	}
 
-	estimate := estimateOutputSize(probe, opts)
+	// Size estimate for the scrubber timeline. A copy remux (video not
+	// re-encoded) lands within container overhead of the source file, so the
+	// source size is a far better estimate than bitrate×duration — use it.
+	// A real transcode re-encodes, so fall back to the bitrate×duration model.
+	var estimate int64
+	switch action {
+	case ActionPassthrough, ActionRemux, ActionRemuxAudio:
+		if fi, statErr := os.Stat(srcPath); statErr == nil {
+			estimate = fi.Size()
+		}
+	}
+	if estimate <= 0 {
+		estimate = estimateOutputSize(probe, opts)
+	}
 
 	t := &transcodeSource{
 		tmpPath:   tmpPath,
@@ -151,6 +164,18 @@ func newTranscodeSource(
 	return t, nil
 }
 
+// NewRemuxSource starts an ffmpeg `-c copy` remux of srcPath into a growing
+// fragmented-MP4 temp file and returns it as a GrowingSource for /stream
+// (hueco #3 / 3b). The video + audio are copied (never re-encoded), so this is
+// only valid when the codecs are already browser-native (h264 + aac) and only
+// the container needs changing — the web's decidePlayMethod enforces that
+// before sending PlayMethod="remux". The browser plays the result progressively
+// via byte-range. Caller MUST Close() it (kills ffmpeg + removes the temp file).
+func NewRemuxSource(ctx context.Context, srcPath string, probe *StreamProbe, ffmpegPath, displayName string) (GrowingSource, error) {
+	opts := TranscodeOpts{Action: ActionRemux, FFmpegPath: ffmpegPath}
+	return newTranscodeSource(ctx, srcPath, probe, ActionRemux, opts, displayName)
+}
+
 // signalNotify wakes any goroutine blocked in ReadAt. Non-blocking: if a
 // notification is already pending the new event is folded into it (callers
 // always re-check size + final after waking, so a coalesced signal still