refactor(moq-net): broadcasts own tracks own groups own frames via inherited Arc<Info>

[kixelated]

Makes the moq-net model a strict ownership chain: a child is born only from its parent's create_*, and each level inherits its parent's Arc<Info> so properties (notably the track timescale) are inherited rather than passed piecemeal.

Ownership: children are born only from parents

• TrackProducer::new and TrackRequest::new are now pub(crate). The only public ways to mint a track are BroadcastProducer::create_track / reserve_track, or a dynamic TrackRequest.
• BroadcastProducer::insert_track is removed. Its one production user, the IETF PUBLISH path in ietf/subscriber.rs, now start_announces the broadcast first and then create_tracks the track from it (so the track is genuinely born from its broadcast), undoing the announce on the error paths.
• Standalone BroadcastInfo::produce() is kept, as agreed. Origin-owned broadcasts (Arc<OriginInfo>) remain a deliberate follow-up.

Inherited Arc<Info> chain

• BroadcastProducer / BroadcastConsumer / BroadcastDynamic hold Arc<BroadcastInfo>, threaded into every track via create_track / reserve_track / TrackRequest::accept. Exposed as TrackProducer::broadcast().
• TrackState holds Option<Arc<TrackInfo>>; each group inherits a shared Arc<TrackInfo> clone (replacing the copied-out Option<Timescale>), and GroupProducer::timescale() reads through it.
• GroupProducer / GroupConsumer hold Arc<GroupInfo>, threaded into every frame. Exposed as FrameProducer::group().

Rename

Group → GroupInfo and Frame → FrameInfo, matching the existing BroadcastInfo / TrackInfo convention. The wire types lite::Group / ietf::Group are unchanged.

Cascade

The only breakage anywhere in the workspace was test-site callers of the now-private constructors (moq-mux ×3 files, moq-json, moq-native test) and qualified moq_net::Group/Frame refs (moq-mux, hang). No production code outside moq-net needed changes. Test sites were migrated to create_track (via a throwaway broadcast helper) and the renamed types.

Public API changes (breaking, targets dev)

• Group renamed to GroupInfo; Frame renamed to FrameInfo.
• TrackProducer::new, TrackRequest::new demoted to pub(crate).
• BroadcastProducer::insert_track removed.
• BroadcastProducer/Consumer/Dynamic now carry Arc<BroadcastInfo>; GroupProducer/Consumer carry Arc<GroupInfo>; FrameProducer carries Arc<GroupInfo>.
• New: TrackProducer::broadcast(), FrameProducer::group().

Testing

• cargo test -p moq-net → 398 lib + 4 doctests pass.
• 62 moq-native loopback broadcast tests pass — validates the IETF publish reshape end-to-end.
• moq-mux / moq-json / hang / moq-relay libs pass.
• cargo build --workspace --all-targets, clippy, and cargo fmt --check (nix toolchain) all clean.

Cross-package sync (follow-up)

• js/net mirror (broadcast.ts / track.ts / group.ts) and doc/concept are not in this PR; they're the tracked follow-up.
• Arc<OriginInfo> (origins owning broadcasts) intentionally deferred.

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(Written by Claude)