feat(moq-net): add OriginDynamic for unannounced fallback broadcasts

[kixelated]

Summary

Adds OriginDynamic, the origin-level analogue of the existing BroadcastDynamic/TrackDynamic on-demand handlers, makes request_broadcast the one public broadcast-lookup, and reworks the FFI surface around it.

Level	Container	On-demand handler
Origin	broadcasts by path	OriginDynamic (new)
Broadcast	tracks by name	BroadcastDynamic
Track	groups by sequence	TrackDynamic

request_broadcast (the unified lookup)

OriginConsumer::request_broadcast(path) -> Result<kio::Pending<BroadcastRequested>, Error> — a kio::Pending future you await (not an async fn), mirroring TrackConsumer::fetch_group. Registration is synchronous (a handler sees the request immediately); the future then resolves:

• announced → immediately with the announced broadcast;
• not announced + live OriginDynamic → once the handler accepts (live broadcast) or rejects (error), coalescing concurrent requests for the same path;
• not announced + no handler → fails fast, synchronously, with NotFound (Dropped once the origin is gone).

Not announced, by design

Dynamically served broadcasts are never announced — OriginConsumer::announced never observes them. The request queue lives in a shared OriginDynamicState off to the side of the announce tree (ref-counted handlers, coalesced requests, reject-on-last-drop). The result rides a one-shot kio channel; kio checks the value before the closed flag, so the outcome is observed without a close race.

Handler side

OriginProducer::dynamic() -> OriginDynamic; requested_broadcast() yields a BroadcastRequest. accept(broadcast) resolves every awaiting requester with the supplied live broadcast (the handler keeps producing into it, e.g. a relay proxying upstream); reject(err) resolves them with an error.

get_broadcast retired from the public API

OriginConsumer::get_broadcast was a synchronous announced-only peek that races announcement gossip. It's now private — the internal helper backing request_broadcast's announced case (and the tree-state test assertions). Every caller moved to request_broadcast:

• moq-net serve paths: lite recv_subscribe, run_track_info, run_fetch, ietf subscribe.
• moq-rtc: WHIP/WHEP gateways.

With no OriginDynamic registered this is behavior-identical (resolve-if-announced, else fail fast); a registered handler adds the fallback. announced_broadcast is unchanged (race-free await-for-announce).

FFI: dynamic broadcast requesting

Neither libmoq nor moq-ffi could previously request a broadcast that hadn't been announced. Now:

• rs/moq-ffi: MoqOriginConsumer::request_broadcast (async).
• rs/libmoq: moq_origin_request + moq_origin_request_close (callback style). Removes the synchronous moq_origin_consume (breaking): it was a racy announced-only peek; moq_origin_request resolves an already-announced broadcast immediately (same common case, no race) and adds the dynamic fallback. Callers reacting to an announcement use the broadcast delivered by moq_origin_announced.
• Wrappers: py (OriginConsumer + Client), swift (OriginConsumer), go (OriginConsumer + Client); kt via the uniffi typealias.
• Docs: py README + doc/lib/py; libmoq README + doc/lib/c.

FFI: free delivered announcements

moq_origin_announced handed its callback a fresh announce ID per event with no way to release one (they accumulated). Adds moq_origin_announced_free to drop a delivered record once read. Explicit free, not auto-on-unannounce: an unannounce is its own record, and auto-freeing the prior one would race a caller still reading its borrowed path pointer.

Scope / follow-ups

• Targets dev (new public surface in rs/moq-net + rs/moq-ffi; get_broadcast privatized; moq_origin_consume removed).
• Follow-up: register an OriginDynamic in the relay/cluster to actually fetch from upstream on a fallback request; js/net mirror; carry the BroadcastConsumer in libmoq's announced record (+ accessor) so announce→consume is fully race-free (now unblocked by moq_origin_announced_free).

Public API changes

rs/moq-net: added OriginProducer::dynamic, OriginConsumer::request_broadcast, OriginDynamic, BroadcastRequest, BroadcastRequested; OriginConsumer::get_broadcast is now private.
rs/moq-ffi: added MoqOriginConsumer::request_broadcast.
rs/libmoq: added moq_origin_request, moq_origin_request_close, moq_origin_announced_free; removed moq_origin_consume.

Test plan

• moq-net: 385 lib tests (7 new for OriginDynamic incl. "never announced")
• request_broadcast fail-fast / served / coalesce / reject / handler-dropped / prefers-announced / clone-keeps-alive
• libmoq announced_free_lifecycle; 7 roundtrip tests migrated onto moq_origin_request
• cargo test -p moq-ffi -p libmoq (24 + 25); moq-rtc / moq-relay build
• cargo fmt --check + clippy --all-targets clean for moq-net / moq-ffi / libmoq / moq-rtc (via nix)

(Written by Claude)