moq-lite-05: implement FETCH for past groups via TrackConsumer::fetch

[kixelated]

What

Implements a one-shot fetch on TrackConsumer, so you can retrieve a single past group without holding a live subscription:

TrackConsumer::fetch(group: u64, options: impl Into<Option<Fetch>>) -> Result<GroupConsumer>
pub struct Fetch { pub priority: u8 }   // defaults priority 0

If the group is already cached it's returned directly; otherwise the request bridges to a wire moq-lite FETCH, blocking on a new FETCH_OK (mirroring how subscribe blocks on SUBSCRIBE_OK). The FETCH was previously a wire-only definition the publisher rejected with UnexpectedStream.

Why

The only way to read a group was to subscribe to a track and walk its cache. There was no first-class way to grab one past group. fetch lives on TrackConsumer (not a live subscription) on purpose: you no longer have to subscribe just to fetch.

How it works

The model→wire bridge reuses the existing dynamic track-request machinery, but for a single group:

TrackConsumer::fetch
  └─ BroadcastConsumer::request_fetch(name, group, Fetch)
       ├─ cache hit (complete/in-progress) → return GroupConsumer
       ├─ cached but aborted → bypass → dynamic group request
       ├─ not cached / no live track → dynamic group request
       └─ no dynamic handler → NotFound
            └─ FetchPending (Future → GroupConsumer)

BroadcastDynamic::requested_group() -> GroupRequest        (relay subscriber drains this)
  └─ GroupRequest::accept(timescale) -> GroupProducer      (filled from the wire)

The FETCH response streams on the same bidirectional control stream as the request (like IETF MoQT): the subscriber writes Fetch; the publisher replies FetchOk then streams the group's frames; stream FIN ends the group. Errors propagate via stream reset.

Changes

• model (model/broadcast.rs): Fetch options struct, a dynamic group-request channel (request_fetch / FetchPending / GroupRequest / BroadcastDynamic::requested_group), and TrackConsumer::fetch. GroupConsumer::timescale() accessor (model/group.rs).
• wire (lite/fetch.rs): new FetchOk (group echo + compression + timescale); Fetch.frame_start (lite-05+). The publisher honors frame_start by skipping earlier frames.
• publisher (lite/publisher.rs): recv_fetch / run_fetch serve the group's frames on the fetch stream (gated to lite-05+).
• subscriber (lite/subscriber.rs): serve group requests by issuing FETCH upstream and routing frames into the producer that resolves the fetcher.
• relay (moq-relay/src/web.rs): GET /fetch/<broadcast>/<track>?group=N uses fetch() instead of subscribe(). ?group=latest keeps the subscribe path (fetch needs a concrete group).

Out of scope (follow-ups)

• frame_start resume reseed. The publisher honors frame_start on the wire, but fetch() always requests 0; an aborted cached group is bypassed with a full re-fetch. True resume (request frame_start = N, stitch onto the N cached frames) needs a GroupProducer that begins at a logical frame offset — a separate model change.
• Compressed fetch. v1 streams fetched groups uncompressed (FetchOk.compression = None); the model GroupConsumer carries no per-track compress hint.
• JS + docs. Cross-Package Sync pairs rs/moq-net wire/API with js/net (which already has a lite/fetch.ts stub) and doc/concept. Deferred to a follow-up.

Built on #1595

#1595 (now merged into dev) added the Fetch.frame_start wire field. dev has been merged in and the fetch.rs overlap reconciled: this PR keeps #1595's field + tests and adds FetchOk plus the serving logic that actually uses frame_start.

Test plan

• cargo test -p moq-net — wire roundtrip (FetchOk, frame_start gating) + 9 model unit tests (cache hit, miss-queues-request, past-final NotFound, aborted-cache bypass, coalescing, no-dynamic NotFound, deny, drop-cancels, dynamic-drop-aborts).
• cargo test -p moq-native --test broadcast — new broadcast_moq_lite_05_fetch_webtransport end-to-end fetch over a real session (WebTransport; Lite05Wip isn't ALPN-advertised so raw QUIC is excluded, matching the other Lite05 tests).
• cargo test -p moq-relay, cargo clippy --workspace --all-targets, cargo fmt --check.

Targets dev per branch targeting (wire-protocol change under rs/moq-net).

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(Written by Claude)