Add CRC32C checksums to WAL record format (v2), implement crash recovery
with automatic truncation of corrupt records, add feature-gated group commit
buffer for batched fsync under concurrent load, and implement log rotation
via segment files with global offset addressing.
Key changes:
- Record format v2: [len:u32][crc32c:u32][blake3:32][payload:N]
- recover_file() scans and truncates corrupt tail records
- GroupCommitBuffer batches fsync via MPSC channel (tokio feature gate)
- SegmentManager with binary search resolution and cursor-based cleanup
- Journal::read() auto-refreshes segments on miss for writer/reader split
- Split recovery.rs and key_codec.rs into directory modules for 500-line max
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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