poc: SWITCH relay implementation (moq-transport PR #1378)

[gwendalsimon]

• Add MoqxSession: thin MoQRelaySession subclass that dispatches onSwitch() to MoqxRelay and exposes publishForSwitch() + writeCatchupToHandle() — both require protected MoQSession access
• Wire MoqxRelayServer::createSession() to produce MoqxSession instances and call setRelay() immediately after construction
• Add SwitchTypes.h: SwitchTransition struct + kSwitchTransitionParamKey
• Add GroupStartObserver.h: lightweight TrackConsumer that fires a callback on each beginSubgroup() for G_switch detection
• Add SwitchHandler: full relay-side state machine (VALIDATE → SUBSCRIBE_TARGET → OBSERVE_GSWITCH → CUT_OLD → CATCHUP+LIVE) including pre-population of availableTarget from forwarder.largest(), inline FETCH catch-up via writeCatchupToHandle(), and Phase 2 drain loop
• Add MoqxRelay::handleSwitch(), getForwarder(), getOrSubscribeForwarder(), getSubscriptionHandle(), cache() to complete the relay glue
• Add moqx_switch_handler_test placeholder target to test/CMakeLists.txt
• Build not yet verified — requires cmake 3.25+ and system deps: run: sudo deps/moxygen/standalone/install-system-deps.sh

---

This change is 

[gwendalsimon]

The moxygen changes are now browsable at: https://github.com/gwendalsimon/moxygen/tree/feat/switch-poc

The moqx implementation is self-contained in this branch. The moxygen submodule carries local-only changes that would need to land in moxygen before this can be merged. Here is what those changes are:

1. MoQTypes.h — SWITCH frame type (0x1B per moq-transport PR #1378), Switch struct, and 4 new PublishDoneStatusCode values (DOES_NOT_EXIST, TIMEOUT, NOT_SUPPORTED, EXCESSIVE_LOAD)
2. MoQFramer.h/cpp — parseSwitch() to decode the SWITCH wire format
3. MoQCodec.h/cpp — onSwitch() virtual callback in ControlCallback, SWITCH allowed on the control stream, dispatch in
   parseFrame()
4. MoQSession.h/cpp — two additions:
   - getBidiStreamConfig() made virtual + a PUBLISH case so relay-initiated PUBLISH bidi streams are accepted by
   moxygen-based subscribers without PROTOCOL_VIOLATION
   - registerPublishConsumerForSwitch() protected factory — registers TrackPublisherImpl in pubTracks_ and
   pendingRequests_ before the PUBLISH frame hits the wire (needed to avoid a PUBLISH_OK race)
5. relay/MoQCache.h — getObject() public accessor for catch-up delivery from the cache