- write_all_sparse: skips all-zero runs >= 4096 bytes via lseek(SEEK_CUR) and
ftruncates the final size, wired into the atomic temp+rename and --inplace
paths with no wire change (full image already in memory).
- --partial retention: on a save failure after the temp held data, rename the
already-written temp to the destination path (best-effort; falls through to
unlink; never retains when --partial is off) so --append/--append-verify can
resume; tested by forcing futimens EINVAL with an out-of-range nsec.
- --block-size aliases --delta-block; verified config->delta_block_size is
honored by the delta engine end-to-end (unit + integration tests).
- fake_super_restore_fd: parses and re-applies user.fastsync.stat fd-relative
(fchown best-effort/non-root skipped, fchmod, futimens); a save under
--fake-super now both records and re-applies.
- -N/--crtimes and --stderr=client promoted to a new 'Impossible/Divergence'
status bucket (Summary: 136/2/4/3/2 = 147).
- cppcheck/clang-format clean; unit 37/37; integration 407 passed.
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