feat(ds-stream): RLE compression, input events, keyframe caching
- protocol.rs: TouchEvent, GamepadAxisEvent, GamepadButtonEvent, ResizeEvent
with encode/decode and roundtrip tests
- codec.rs: rle_encode/rle_decode for delta frame compression (333x on
unchanged frames), signal_sync_frame/signal_diff_frame builders,
touch/gamepad/stream_end convenience builders
- relay.rs: StateCache for late-joining receivers (keyframe + signal sync +
accumulated diffs), periodic keepalive pings, stats logging every 30s,
diff accumulator with cap at 1000
- BITSTREAM_INTEGRATION.md: compiler integration spec for another agent
- Tests: 17 → 38 (all passing)
69f39746af