v0.4.8rc2-dev31

Engine-driven multi-group transfer optimizations

Wire-Format (Tier 1, always on)

• Replaces pickle.HIGHEST_PROTOCOL with torch.save on the worker→server wire
• b'L' magic byte + payload; legacy pickle blobs auto-detected on first byte
• torch.save reaches ~1.6-1.8 GiB/s on production-sized (~50 MiB) bf16 tensors
  vs pickle's ~0.9 GiB/s -- ~2x speedup on serialize + deserialize
• Cumulative effect on the 14 GiB / 4-group workload: ~1.5-2s shaved per store

Multi-Stream D2D (Tier 1, opt-in)

• LMCACHE_MP_MULTI_STREAM_D2D=1 runs each group's GPU→CPU gather on its own
  CUDA stream
• Default is OFF because on consumer GPUs the PCIe bus is shared and
  per-stream sync overhead exceeds the overlap win (4x slower in our test)
• Available for A/B testing on different hardware

Cleanup

• Removed duplicate _serialize_single_group_chunks (was defined twice)
• Removed dead _to_serializable_chunk and tagged-tuple wire format
• Removed duplicate _pinned_pool initialization

Compatibility

• Backward-compatible wire format: dev<=30 pickle blobs still decode
• No L2 cache wipe required when upgrading
• Same COMMIT_STORE_GROUP request shape; the new format is auto-detected