v0.4.8rc2-dev25

Performance: skip bf16→float32 conversion + pickle.HIGHEST_PROTOCOL.

Wire-format compatibility: backward-compatible deserializer for blobs
written by dev15-dev23. No L2 cache wipe required when upgrading from
any earlier wheel.

Benchmarks (bf16, 10 chunks ~640 MiB):
OLD bf16→float32: 1314 ms, 1280 MiB blob
NEW pickle HIGHEST: 704 ms, 640 MiB blob
Speedup: 1.87×, 50% smaller blob

Includes all earlier fixes:

• msgspec 4 GiB bin limit fix via per-group COMMIT_STORE_GROUP messages
• fp8 dtype support (e4m3fn, e5m2)
• assert → ValueError replacement
• group count validation