moq-net: reshape Track into TrackInfo + an async TrackConsumer handle

[kixelated]

Summary

Reworks the track model so we don't have to know TrackInfo until a SUBSCRIBE_OK / FETCH_OK / TRACK_STATUS arrives, which makes room for FETCH alongside SUBSCRIBE. TrackConsumer is now a cheap async handle you subscribe/info (later fetch); behind the scenes that resolves into a SUBSCRIBE, and TrackRequest::accept(info) locks the info in and returns a static TrackProducer. Completes the prototype and updates every consumer/test across the workspace.

Model (rs/moq-net)

• Split Track (name + properties) into TrackInfo (properties only); the name now lives on the handles. Broadcast → BroadcastInfo.
• BroadcastConsumer::consume_track → fallible track(name). Subscribing is track(name)?.subscribe(sub)?.await, resolving on SUBSCRIBE_OK. TrackRequest::accept(info) hands back a TrackProducer that can't change the info.
• Added TrackProducer::{consume, subscribe} (synchronous in-process subscribe, since info is known at creation) and sync subscription() snapshots; the async change-notifier is now subscription_changed().
• ⚠️ As noted in the prototype, moq-lite has no TRACK_STATUS yet, so a standalone info() would block — older versions still go through a real subscribe. Left as-is for now.

Deadlock fixes found wiring it up end-to-end

Both surfaced via stack-sampling the hung moq-ffi::server_client_roundtrip:

• Lost wakeup on accept() — TrackSubscriberPending/TrackInfoPending passed a temporary kio::Waiter; kio stores a weak waker, so it died on poll return and accept()'s wake hit nobody. Now the waiter is held across polls.
• Spurious modified — poll_subscription_changed used retain_mut, tripping Mut's DerefMut and flagging TrackState modified on every no-op poll (draining unrelated waiters). Now a shared helper iterates immutably; added kio::Consumer::is_closed.

Catalog (rs/hang)

• The catalog embedded moq_net::Track, which no longer carries a name and shouldn't carry compress/timescale/cache anyway. Replaced with a { name: String } type matching the JS TrackSchema exactly (byte-identical wire, no js/hang change). default_track() → default_track_info() -> TrackInfo (it only ever configured the catalog.json producer).

Allocation

• Track names are Arc<str> (handle clones — weak/consume/subscribe/accept — are refcount bumps; impl Into<Arc<str>> lets an existing Arc pass through free). The broadcast lookup maps key by the same shared Arc<str>. TrackInfo stays a plain Clone value (small, inline, no heap).

Consumers updated

hang, moq-mux, moq-relay, moq-audio, moq-cli, moq-gst, moq-boy, moq-rtc, libmoq, moq-ffi, moq-native (+ their tests/examples).

Test plan

• cargo test --workspace — 941 passed, 0 failed (2 ignored)
• cargo fmt --all --check clean (Nix-pinned toolchain)
• Previously-hung moq-ffi::server_client_roundtrip passes
• js/ not touched — this is an API-shape change, not a wire change (catalog JSON is identical); JS @moq/net handle parity is a possible follow-up

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(Written by Claude)