Add DatPathLen (2 bytes at offset 24) + variable-length DatPath after
the 32-byte header. Solves the filename mismatch where sra-volume
assumed {vol}.dat but SeaweedFS uses {collection}_{vol}.dat.
Backward compatible: old clients that only read 32 bytes still work
(DatPathLen is in the formerly-reserved region, and the trailing
path bytes are simply ignored by old readers).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
pingqiuandClaude Opus 4.6
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