feat(moq-gst): relay-less listen mode for moqsink and moqsrc + live catalog subscription

[santi-ferreiro]

Summary

Enables relay-less MoQ in moq-gst by letting either element run its own QUIC/WebTransport server, and fixes a catalog race in moqsrc that this exposed. Supersedes #14 (includes its commits; this targets benchmark-develop directly).

1. moqsink listen mode — listen + tls-generate properties; the sink runs a server and serves its broadcast to subscribers that dial it (publisher-as-server topology, used by the group-ordering verification).
2. moqsrc listen mode — the mirror: the source runs a server and consumes the broadcast from a publisher that dials it (subscriber-as-server topology). This keeps the publisher as the QUIC client, which is required for IP-migration testing since QUIC connection migration is client-only.
3. moqsrc live catalog subscription (bug fix, all topologies) — moqsrc now keeps consuming catalog updates for the life of the session and subscribes to tracks as they appear, instead of acting on a single startup snapshot.

Listen mode

Both elements gain a Transport { Client { url } | Server { bind, tls_generate } } resolution; listen is mutually exclusive with url and dial mode stays the default. The sink's server branch mirrors the relay accept-and-serve path (request.with_publish); the source's mirrors it on the consume side (request.with_consume(origin).ok().await). Everything downstream of session establishment is unchanged in both elements.

Catalog fix

The catalog is a live track that mutates during publisher startup: audio registers at caps time, video only at the first keyframe, and a track replacement briefly publishes an empty catalog (the old importer's Drop removes its rendition before the successor re-adds it under a new name).

moqsrc previously read one catalog snapshot. Depending on timing it could see zero tracks (and exit as if the stream had ended: a silent, unrecoverable stall) or an audio-only list (never subscribing to video). The race is latent in every topology; subscriber-listen exposes it because the subscriber is connected and reading before the publisher's pipeline warms up.

Fix: run_session loops on catalog_updates.next(), subscribing to each new track (deduplicated by name) until the catalog track finishes or shutdown. A dead session now surfaces as a gst::element_error! instead of idling forever. The now-invalid NoMorePads message is removed since pads can appear at any point in the stream.

Testing

• Subscriber-listen topology (paired benchmark-repo PR): full sequential suite 7/7, previously stalling on 5/7; IP-switch test shows genuine QUIC migration (PATH_CHALLENGE/PATH_RESPONSE on the same connection, packet-level trace).
• Dial-mode topologies (relay, publisher-listen) re-verified: stable and packet-loss runs unchanged, zero frame loss on paths that already worked.

Cross-package sync

• doc/bin/gstreamer.md — listen-mode sections for both elements.

(Written by Claude)

[gemini-code-assist]

Code Review

This pull request introduces a listen mode for both moqsink and moqsrc GStreamer elements, allowing them to run a local QUIC/WebTransport server and serve or consume broadcasts directly without a relay. It also updates moqsrc to dynamically handle catalog updates and subscribe to tracks as they appear. A critical issue was identified in moqsrc's listen mode where the server's accept loop does not select on the shutdown signal, potentially causing the pipeline shutdown to hang indefinitely if no publisher connects.

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[gemini-code-assist]

In moqsrc listen mode, server.accept().await is called without selecting on the shutdown signal. If the element is stopped before a publisher connects, the task will hang indefinitely on this await, causing the GStreamer pipeline shutdown to stall.

We should wrap server.accept() in a tokio::select! with shutdown.changed() to ensure graceful termination.

			let request = tokio::select! {
				res = server.accept() => res.context("listener closed before a session arrived")?,
				_ = shutdown.changed() => return Ok(()),
			};

[santi-ferreiro]

Good catch — fixed in af73560. server.accept() is now wrapped in a tokio::select! with shutdown.changed(), matching how the dial path already guards the announce wait. Verified the accept path still works end-to-end after the change. (Written by Claude)

[santi-ferreiro]

Consolidated into #14 — this branch's commits were fast-forwarded onto feat/moqsink-listen-mode, so #14 now contains the moqsrc listen mode, the catalog fix, and the shutdown-select review fix. Closing to keep a single review. (Written by Claude)