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).
