feat: Replace ANNOUNCE -> (PUBLISH_NAMESPACE + PUBLISH) and SUBSCRIBE_NAMESPACE

[itzmanish]

Summary
Replaces the legacy ANNOUNCE-based namespace advertisement and subscription routing with the draft-16 aligned PUBLISH_NAMESPACE, PUBLISH, and SUBSCRIBE_NAMESPACE message flows. This is a major protocol evolution that decouples namespace advertisement from subscription routing, enabling both push-based (PUBLISH) and pull-based (SUBSCRIBE) media delivery through the relay.

Motivation
The ANNOUNCE mechanism in earlier drafts bundled namespace advertisement with subscription routing — when a publisher announced a namespace, incoming SUBSCRIBEs were routed through the Announce object directly. Draft-16 separates these concerns:

• PUBLISH_NAMESPACE — advertises that a publisher has tracks available in a namespace (replaces ANNOUNCE)
• SUBSCRIBE — pull-based: subscriber requests data, relay forwards upstream to publisher
• PUBLISH — push-based: publisher proactively sends track data to the relay
• SUBSCRIBE_NAMESPACE — subscriber discovers available namespaces from a publisher
  This separation enables more flexible relay architectures where publishers and subscribers can use different delivery strategies for the same content.

Changes

Protocol Layer (moq-transport/src/session/)

• Removed: announce.rs (Announce/AnnounceRecv) — replaced by decoupled message handlers
• Added: publish_namespace.rs — outbound PUBLISH_NAMESPACE (publisher → relay)
• Added: publish_namespace_received.rs — inbound PUBLISH_NAMESPACE handler (renamed from announced.rs)
• Added: published.rs — outbound PUBLISH with full subgroup/datagram serving (publisher → relay)
• Added: publish_received.rs — inbound PUBLISH handler (relay side)
• Added: subscribe_namespace.rs — outbound SUBSCRIBE_NAMESPACE
• Added: subscribe_namespace_received.rs — inbound SUBSCRIBE_NAMESPACE handler
• Modified: publisher.rs — removed announce(), added publish_namespace(), publish(), subscribe_namespace_received() APIs; all SUBSCRIBEs now route through unknown_subscribed queue
• Modified: subscriber.rs — added PUBLISH message handling, publish alias tracking
• Modified: subscribed.rs — added is_closed() cancellation checks in serve paths
• Modified: mlog events for PUBLISH/PUBLISH_OK/PUBLISH_ERROR/PUBLISH_DONE

Relay (moq-relay-ietf/)

• local.rs: Complete rewrite — introduced TrackInfo state machine (Pending → Subscribing → Subscribed → Publishing → Closed) and Locals registry with LocalsEntry containing TracksReader, TracksWriter, and per-track HashMap<String, Arc<TrackInfo>> for concurrent push/pull management
• producer.rs: serve_subscribe() now uses TrackInfo system — calls get_or_create_track_info(), checks should_subscribe_upstream(), forwards via subscribe_upstream(). Added serve_subscribe_namespace() handler
• consumer.rs: serve() renamed to serve_publish_namespace() — registers both reader AND writer with Locals. Added serve_publish() for handling inbound PUBLISH messages (push-based delivery)

Serve Layer (moq-transport/src/serve/)

• tracks.rs: Added forward_upstream() on TracksReader, insert() and remove() on TracksWriter for relay track management
• track.rs: Added is_closed(), largest_location(), comprehensive test suite for stale track cache eviction
• subgroup.rs/stream.rs/datagram.rs: Added is_closed() checks

Clients

• moq-pub: Switched from publisher.announce(reader) to publisher.publish_namespace(namespace) + explicit serve_subscriptions() loop
• moq-clock-ietf: Updated to use publish_namespace() API
• moq-test-client: Updated scenarios for PUBLISH message flow

Bug Fixes Included

• fix: prevent moof/mdat interleaving in moq-sub — The new relay architecture (shared TrackInfo with OnceLock + inline forward_upstream) delivers track data with tighter timing than the old announce-based path, which increases the probability of concurrent audio/video stdout writes interleaving between moof and mdat boxes. This caused H.264 decode errors (Invalid NAL unit size where 0x74726166 = "traf" box type was parsed as NAL data). Fixed by buffering moof and writing moof+mdat as a single atomic operation in moq-sub's recv_group().
• fix: detect and evict stale cached TrackReaders — TracksReader::subscribe() now checks is_closed() before returning cached tracks, evicting stale entries and requesting fresh ones from the publisher. Prevents subscribers from being stuck on dead tracks after publisher reconnection.
• fix: is_closed() method on Track — Properly checks all states (TrackWriter dropped, SubgroupsWriter dropped, closed with error) to determine if a track is truly closed.
• fix: race between publish and subscribe — TrackInfo state machine prevents conflicts when both PUBLISH and SUBSCRIBE arrive for the same track.

Testing

• Unit tests for stale track cache eviction (test_stale_track_cache_bug)
• Unit tests for track deduplication (test_track_deduplication_while_alive)
• Unit tests for subgroup writer lifecycle (test_track_not_stale_after_subgroups_transition, test_track_stale_after_subgroups_writer_dropped)
• Updated moq-test-client scenarios