- CLI: --append/--append-verify acceptance (parse + imply --incremental,
validate) and incompatibility rejection with -s and --whole-file; both
removed from the unimplemented reject list.
- Config: on-the-wire append/append_verify round-trip.
- Unit: append_resume_eligible / append_tail_length pure resume math.
- Integration (test_append.py): matching-prefix resume is byte-identical and
tail-only (wire bytes << source size); --append with a wrong prefix keeps
prefix+tail (rsync parity) while --append-verify detects the mismatch and
falls back to a byte-exact full transfer; --append with --inplace and -m.
