fix(storage): harden zero-size idx scrub and mirror to rust
When a zero-size .idx is found, openIndex stats the backing .dat through
v.DataBackend: wrap that GetStat failure with %w, fix the indices typo, and
guard both openIndex and scrubVolumeData against a nil DataBackend (closed or
remote-only volumes) instead of panicking.
Add rust scrub tests for empty (superblock-only .dat, zero-size .idx) and
healthy volumes, keeping the volume server in parity with the go zero-size
scrub handling.
Chris Lu
5b42287c22