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. (Originally moqsink-only; expanded to include the moqsrc mirror and the catalog fix.)

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), with the accept guarded by the element shutdown signal. 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 qualabs/moq-benchmark#51): 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 moqsink in the GStreamer plugin, allowing it to run its own QUIC/WebTransport server and serve broadcasts directly to subscribers instead of dialing a relay. It adds configuration properties listen and tls-generate to support this mode. The review feedback highlights several important improvements: validating that tls-generate is provided when in listen mode to prevent runtime initialization failures, implementing a shutdown signal to prevent active accepted sessions from leaking when the element is stopped, and correcting a discrepancy in the documentation regarding the mutual exclusivity of the listen and url properties.

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