test(parity): add differential rsync-parity CI gate

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.

AGENTS.md

@@ -56,8 +56,15 @@ cmake -B build -S . && cmake --build build -j$(nproc)
 ./build/tests                # unit tests
 python3 -m pytest tests/integration/ -n 4 --dist=load -m "not setpriv"   # full integration suite (CI excludes env-dependent privilege tests)
 python3 -m pytest tests/integration/ -n 4 --dist=load -m ci   # PR-gate subset only
+
+# Differential rsync-parity gate (real rsync 3.4.1 vs FastSync)
+python3 -m pytest tests/integration/test_differential_parity.py -n 4 --dist=load -m parity_ci  # fast PR subset
+python3 -m pytest tests/integration/test_differential_parity.py -n 4 --dist=load -m parity     # full set
 ```
 
+See `tests/integration/README.md` for the differential parity gate and its
+`parity_caveats.py` allowlist (the residual burn-down mechanism).
+
 ## CI Workflow — Waiting for Results
 
 When running the CI workflow via `tea` (the task execution agent), always set a sufficient timeout (e.g., 600000ms) to allow CI to finish. After CI completes, check the results yourself — do not assume success. Monitor CI status via the Gitea API (see below) or `tea actions`, then inspect logs on failure.