Wire livekit-datatrack Rust managers into the Swift SDK's Room,
SignalClient, and transport infrastructure. Data tracks provide
frame-oriented, real-time data delivery with built-in DTP packetization
and optional E2EE.

Scaffolding (Phase 1):
- Forward raw WebSocket bytes to Rust managers for signal routing
- Create _data_track publisher/subscriber WebRTC data channels
- Delegate bridges for signal requests, DTP packets, and track events
- Manager lifecycle tied to Room connect/disconnect/reconnect
- DataTrackDelegate protocol for track published/unpublished events

Public API (Phase 2):
- LocalParticipant.publishDataTrack(name:) and withDataTrack(name:body:)
- LocalDataTrack.send(contentsOf:) for piping AsyncSequence to a track
- AsyncPolling protocol with .values for DataTrackStream iteration
- DataTrackFrame convenience extensions (.now, .latency)

E2E Tests (Phase 3):
- 8 tests mirroring Rust data_track_test.rs (publish/receive, large
  frames, duplicate name, unauthorized, state, timestamp, resubscribe,
  many tracks)
- Test helper Room.waitForDataTrack(name:) for async track discovery

Tests require livekit-server with enable_data_tracks and a tokio runtime
fix in livekit-uniffi (pending).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Forward serialized protobuf bytes after parsing (not raw WebSocket
  bytes) so JSON-encoded messages from the server are also forwarded
  to Rust data track managers
- Register DataTrackWatcher before publishing to avoid missing the
  initial ParticipantUpdate event
- Use AsyncStream-based watcher for reliable async track discovery
- Simplify resubscribe test to 2 iterations with delay

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Upstream livekit-uniffi replaced the generic `handleSignalResponse`
with specific per-message handlers:
- handleSfuRequestResponse (for RequestResponse)
- handleSfuPublishResponse (for PublishDataTrackResponse)
- handleSfuParticipantUpdate (for ParticipantUpdate)
- handleSubscriberHandles (for DataTrackSubscriberHandles)

Each handler returns UnsupportedType for messages it doesn't handle,
so the simplest integration is to call all four with the raw bytes
and let them filter internally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>