perf: streaming core improvements — batched diffs, RTT tracking, relay merging
Runtime improvements:
- Diff batching: multiple signal changes coalesced into 1 WS frame
via _pendingDiffs + microtask Promise.resolve()
- Connection status: _streamConnected, _streamLatency, _streamReconnects,
_streamFrameCount, _streamByteCount tracked for stream health
- RTT tracking: periodic ping/pong every 5s measures round-trip latency
- Removed redundant _streamSync from flush() — diffs are already batched
Relay improvements:
- Diff merging: late joiners receive 1 consolidated sync frame instead
of replaying hundreds of individual diffs
- Version counters merged: conflict resolution preserved across catchup
- Fallback: if JSON parse fails, falls back to raw sync + all diffs
Test updates:
- state_cache_signal_sync verifies merged payload (count: 1)
- All 54 relay tests pass
- All 8 examples pass regression
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