Relay a live source at the source boundary, byte-exact

[ralyodio]

Follows #124 (merged), which shipped the relay compression baseline with the source boundary documented but unbuilt. A live channel could only be relayed as its post-ffmpeg output, and asking for the source bytes always answered SOURCE_BOUNDARY_UNAVAILABLE. This builds the source boundary for the sources nixamp can read exactly.

What it does

• A channel can read its own source and pipe it to ffmpeg instead of letting ffmpeg dial it. Every chunk down that pipe is handed to the source's taps first, so a tap sees exactly the bytes ffmpeg does: the original transport stream, padding and all, not the re-muxed output that drops null packets. The tap ends when the source starts over, and the pipe is paced by ffmpeg's own reading through ordinary backpressure.
• Only exactly-readable sources are read this way: a transport stream from a plain http(s) URL or a local file, joined from its start, with no per-request headers and no separate audio file. Anything else stays ffmpeg's to dial, and a source-boundary relay of it is refused with the reason. The readability gate is one tested function; it treats ffmpeg transport demux tuning (-probesize, -analyzeduration, -fflags, as Raw 1080p and 4K transport streams play, go live, and reach a browser #126 now passes for every TS) as fine, and rejects only real request-header flags.
• The envelope and negotiation now carry the boundary and the media codecs. A source relay taps the source; a channel relay listens to the output; the two are different variants, so a channel can serve both at once without one compressor doing the other's work. A receiver learns the boundary and codecs before the first byte, so a source-boundary receiver hands the bytes to its own ffmpeg without a probe.
• Bringing a relay in probes it first: a source-boundary upstream is read through this server too, so it plays here and can be relayed on again at either boundary.
• A source-boundary relay is a live join with no preface: the receiver's demuxer re-syncs on the next program table and keyframe, like any live join.

Tests (9, all passing)

• the readability gate against each way a source fails it;
• a deterministic byte-exact tap with a controlled source and a stub ffmpeg, including rollover ending the tap;
• an end-to-end source relay over HTTP that reassembles the original packets byte for byte from the start;
• a real-ffmpeg tee reading a transport-stream file;
• and the existing route/negotiation suite.

Full suite: 545 pass, the one remaining failure is the pre-existing @profullstack/player web-test module resolution, unchanged from main.

Not here

Sources ffmpeg alone can reach — behind per-request headers (a yt-dlp link), a separate audio track, or a non-transport container — stay channel-boundary only, as documented. docs/stream-compression.md is updated for all of the above.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MxNif5tsYq4LczgG7aE8Jp