Remove BLAKE3 prefix digests. Keep READY/FINISH as empty records since
they're semantically important markers.
Our existing format (CRC32 per-record, declared counts, strict tag
ordering requirements, etc.) already detect: accidental corruption,
truncation, data omission, and duplication.
BLAKE3 only protects against valid records being swapped or removed
entirely. It is heavy for just that, and callers can solve that anyways
via their own transport (like, just use TCP). For more adversarial
protection, callers can also add layers like TLS or their own alternate
signing methods depending on their own threat models.
Removing the hash improves encode times by ~1.4x, decode times by ~1.3x.
Time-to-READY decoding is effectively unchanged because it was such a
small package to begin with.
**AI usage:** I had it clean up the comments and the tests, but I did
the blake3 removal and marker changes, and wrote the commit message
myself. All reviewed.
204989ad90