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.

tests/integration/parity_caveats.py

@@ -0,0 +1,80 @@
+"""Data-driven allowlist for the differential rsync-parity gate.
+
+Every entry maps a case id (see ``test_differential_parity.py``) to the aspects
+that are *known* to differ from ``rsync 3.4.1`` and the documented reason.  A
+differential mismatch in an aspect that is **not** listed here fails the gate.
+
+Aspect keys
+-----------
+``tree``    destination tree differs (paths, file hashes, symlink targets,
+            modes, hardlink grouping)
+``stdout``  normalized output for ``-i`` / ``--stats`` / ``--out-format``
+``extra``   a case-specific assertion differs (basis/inode checks, ...)
+``rc``      exit status differs
+
+Burn-down
+---------
+If a case is listed here but now matches rsync, the gate emits a loud
+``pytest`` warning naming the stale entry: delete the entry (and, when the
+underlying row in ``RSYNC_COMPAT.md`` is now parity, update that row).  Set
+``FASTSYNC_PARITY_STRICT=1`` to turn stale entries into failures in CI.
+
+Keep the values concise but cite the governing row so the entry can be
+re-triaged when the row moves.
+"""
+
+# case id -> {aspect: "reason (ref: RSYNC_COMPAT.md ...)"}
+CAVEATS = {
+    # A source subtree whose only files are all filtered out (here, by
+    # --min-size) is left behind as an empty directory by rsync but not by
+    # FastSync: the recursive scanner keeps directory entries record-only, so
+    # `sub/deep` is only implicit through the (skipped) file.  This is the
+    # documented recursive-empty-directory residual, not a payload/selection
+    # bug.
+    "min_size": {
+        "tree": "recursive transfer does not create a source directory that "
+                "becomes empty after --min-size filtering (FastSync directory "
+                "entries are record-only). ref: RSYNC_COMPAT.md `-d/--dirs` "
+                "row and completion-wave residual ('recursive transfers still "
+                "do not create empty directories').",
+    },
+    # FastSync's recursive scanner keeps directory entries record-only, so a
+    # plain `-d`/recursive source whose only role for a directory is that entry
+    # (empty dir, or a dir emptied by filtering) is not created on the
+    # destination.  rsync creates it.  `--dirs`/`--files-from`-listed
+    # directories DO cross as explicit entries (covered by the passing
+    # `empty_dirs_files_from` / `files_from` cases).
+    "empty_dirs_recursive": {
+        "tree": "recursive transfer does not create empty source directories. "
+                "ref: RSYNC_COMPAT.md `-d/--dirs` row and completion-wave "
+                "residual ('recursive transfers still do not create empty "
+                "directories').",
+    },
+    # A plain `-d` invocation: FastSync's `--source-dir` treats the argument as
+    # the directory entry itself (creates the empty source-root mirror), while
+    # rsync's `src/` trailing-slash form lists the immediate contents.
+    "dirs_plain": {
+        "tree": "plain -d semantics: FastSync creates the source-root directory "
+                "entry (its documented --dirs files-from behavior) instead of "
+                "rsync's one-level contents listing for a `src/` argument. "
+                "ref: RSYNC_COMPAT.md `-d/--dirs` row (⚠️).",
+    },
+    # --max-delete stops the extras walk part-way and exits 25 in both
+    # implementations; which of the remaining extras survives depends on
+    # deletion order, which neither tool specifies.  The exit code and the
+    # number of survivors match (asserted implicitly by the harness's rc
+    # comparison and the one-for-one diff below).
+    "max_delete": {
+        "tree": "which destination extras survive a partial --max-delete abort "
+                "is deletion-order dependent and unspecified; rc=25 and the "
+                "number of survivors match rsync. ref: RSYNC_COMPAT.md "
+                "`--max-delete=NUM` row.",
+    },
+}
+
+# Accepted aspect names (guards against typos in this file).
+ASPECTS = ("tree", "stdout", "extra", "rc")
+
+
+def caveat_for(case_id: str) -> dict:
+    return CAVEATS.get(case_id, {})