Add tests/integration/test_differential_parity.py plus a shared
parity_harness.py and a data-driven parity_caveats.py allowlist. The gate
runs real rsync 3.4.1 and FastSync over the same corpora, compares the
destination trees (paths, hashes, symlink targets, modes, hard-link
grouping) and the normalized -i/--stats/--out-format output, and fails on
any difference not listed in the allowlist. Stale allowlist entries warn
(or fail under FASTSYNC_PARITY_STRICT=1) so the residual list shrinks.
Register parity/parity_ci markers and wire a fast PR job (parity_ci) plus a
push-only full job (parity, strict) into .gitea/workflows/ci.yaml.
Document the gate and the allowlist workflow in tests/integration/README.md.
