KarpelesLab / tmpsecfile / 24973400414

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Add isSparse flag to skip per-block zero check when no holes exist

Files written sequentially never have a sparse block on disk, so the
isZero scan in ReadAt is wasted work. Track an isSparse bit that
WriteAt and Truncate flip on only when they actually leave an
untouched block past the previously rounded length, and gate the
sparse check on it.

Adds a table-driven test covering: fresh file, contiguous write,
unaligned write within the same rounded block, write that skips a
full block, truncate-extend, and truncate-shrink.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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