* fix(volume): verify the .dat-tail needle in the integrity check
CheckVolumeDataIntegrity checked the last entry by file position in the .idx
and, for a live needle, flipped the volume read-only when fileSize > fileTailOffset.
That entry is the .dat tail only when the .idx is in append order; a key-sorted
.idx (weed fix and other rebuilds listed entries by key) puts the highest-key
needle last, whose tail sits mid-file, so healthy volumes went read-only on every
load and re-running weed fix only reproduced the sorted index.
Locate the needle at the maximum offset — the one physically last in the .dat —
and verify the .dat ends exactly at it, regardless of .idx ordering. The
append-ordered common case stays O(1) (the last entry's on-disk end matches the
.dat size); only a key-sorted index pays a single linear scan. Deletion
tombstones at the tail are now verified too, instead of skipping the file-size
check.
* fix(command): weed fix rebuilds the .idx in .dat offset order
SaveToIdx wrote entries via AscendingVisit — sorted by key, the .sdx/.ecx shape
— so the rebuilt .idx put the highest-key needle last instead of the .dat-tail
needle, and dropped tombstones whose live needle was gone. Collect the live and
deleted entries, sort by .dat offset, and write them in append order so the .idx
stays a faithful log whose last entry is the real .dat tail.
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