feat(moq-net): request_broadcast returns Unroutable when no route exists

[kixelated]

Summary

Follow-up to #1772 (the new OriginConsumer::request_broadcast). Two asks:

1. Return Unroutable instead of NotFound. request_broadcast previously failed with Error::NotFound when a path was neither announced nor served by an OriginDynamic handler. That's indistinguishable from a track/broadcast that genuinely doesn't exist, so this adds a dedicated Error::Unroutable (wire code 30) meaning "no announcement and no dynamic router, so there is no route to this broadcast." It is returned in both failure modes:

   • synchronously, when no handler is live (dynamic == 0);
   • asynchronously, when a request was registered while a handler was live but then loses every handler before being served (the channel closes without a result).

   An explicit BroadcastRequest::reject(err) still surfaces the handler's own error unchanged.

2. Checked the "accept before dynamic" race. Conclusion: the model is race-free. OriginDynamic::new/clone/drop and the dynamic == 0 check in request_broadcast all mutate the count under the same lock, so a handler that is live when request_broadcast acquires the lock always registers the request rather than rejecting it. Crucially, accept is decoupled from the count: once a handler has picked a request up via requested_broadcast, the BroadcastRequest owns the result channel, so it can still accept and resolve the requester even after every handler (itself included) drops and flips the count to zero. Added dynamic_request_accept_after_handler_dropped to lock that in.

   The only remaining race is caller-ordering: if request_broadcast runs before a handler is created, it fails fast with Unroutable. That's the same race the track-level code already addresses by creating the dynamic handler before announcing (see lite/subscriber.rs:414), and the synchronous increment in OriginDynamic::new makes the same "create before expose" discipline sufficient here. No relay site wires OriginDynamic in yet, so there's no caller to fix today.

Public API changes

• moq-net: new enum variant Error::Unroutable (the enum is #[non_exhaustive], so additive). New wire code 30. libmoq already maps every moq_net::Error to a single -2 status, so no FFI change is needed.

Test plan

• cargo test -p moq-net --lib (386 passed)
• cargo clippy -p moq-net clean
• dynamic_request_unroutable_without_handler (renamed) asserts Unroutable
• dynamic_request_handler_dropped asserts Unroutable for both the pending future and a fresh request
• dynamic_request_accept_after_handler_dropped (new) covers accept-after-drop

(Written by Claude)

🤖 Generated with Claude Code

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