fix(p7-times): dir-time entries only record (never create dirs); bound/chunk dir-time frames; harden list add; docs+tests

Review fixes for Phase 7 Wave D.

#1 (HIGH): STATUS_DIR_TIMES entries no longer create directories. A new
receiver-only File.dir_time_only flag marks dir-time entries; file_save_to_disk_full
short-circuits them as FILE_SAVE_SKIPPED before any device/dir branch, so the sink
still accumulates metadata into the deferred DirTimeList but creates nothing. Empty
source dirs stay untransferred (-a), -m/--prune-empty-dirs semantics are preserved,
and a pre-existing regular file/symlink at an empty-dir mirror path no longer aborts
the transfer. dir_time_list_apply fstatat()s the leaf (AT_SYMLINK_NOFOLLOW) and skips
absent/non-directory paths QUIETLY; only a real existing directory is stamped.
Also initialize File.dir_time_only in file_create() (uninitialised garbage otherwise).

#2 (MED): send_dir_times() chunks entries into repeated STATUS_DIR_TIMES frames of at
most MAX_MANIFEST_ENTRIES, matching the receiver's per-frame bound; the tautological
> INT_MAX check is gone.

#3 (LOW): dir_time_list_add() assigns each grown array right after its realloc (no
dangling) and advances capacity only after both succeed.

#4 (LOW): RSYNC_COMPAT.md -- STATUS_MKDIR carries metadata, dir times are transmitted
via STATUS_DIR_TIMES and applied at the end, empty dirs are still never created; -m
rationale, -O row and Wave D notes updated. Summary counts untouched.

#5 (LOW): integration tests for the three #1 scenarios (empty-dir non-creation under
-a and -a -m, collision non-abort), scanner test now covers empty-dir capture, and
test_file_restore_symlink_metadata asserts the positive apply path when supported.

PROTOCOL_VERSION stays 2.17.0; config-frame layout unchanged.

tests/integration/test_features.py

@@ -4797,3 +4797,72 @@ class TestDirectoryAndSymlinkTimes:
             "-J must suppress symlink times"
         assert abs(os.stat(os.path.join(recv_j, "sub")).st_mtime - DISTINCT_MTIME) < 2, \
             "-J must not suppress directory times"
+
+    @pytest.mark.ci
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_preserve_does_not_create_empty_source_dir(self, shared_server, mt):
+        """P7 Wave D #1: a captured-but-EMPTY source directory is never created
+        at the destination.  The scanner records its time (it is transmitted via
+        STATUS_DIR_TIMES), but the receiver treats that entry as record-only, so
+        `-a` keeps the documented "empty dirs are never transferred" behavior."""
+        source = os.path.join(TEST_DATA_DIR, f"empty_dir_{'m' if mt else 's'}_src")
+        dest = os.path.join(TEST_DATA_DIR, f"empty_dir_{'m' if mt else 's'}_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        with open(os.path.join(source, "keep.txt"), "wb") as fh:
+            fh.write(b"regular file\n")
+        os.makedirs(os.path.join(source, "empty_sub"))
+        flags = ["-a"] + (["--threads"] if mt else [])
+        received = self._run(source, dest, flags, shared_server)
+        assert os.path.isfile(os.path.join(received, "keep.txt")), "regular file missing"
+        assert not os.path.lexists(os.path.join(received, "empty_sub")), \
+            f"-a created an empty source directory at {received}/empty_sub"
+
+    @pytest.mark.ci
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_prune_empty_dirs_still_does_not_create_empty_dir(self, shared_server, mt):
+        """P7 Wave D #1: `-a -m` (--prune-empty-dirs) keeps its semantics -- a
+        captured empty directory is never created even though its time is
+        recorded."""
+        source = os.path.join(TEST_DATA_DIR, f"prune_empty_{'m' if mt else 's'}_src")
+        dest = os.path.join(TEST_DATA_DIR, f"prune_empty_{'m' if mt else 's'}_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        with open(os.path.join(source, "keep.txt"), "wb") as fh:
+            fh.write(b"regular file\n")
+        os.makedirs(os.path.join(source, "empty_sub"))
+        flags = ["-a", "-m"] + (["--threads"] if mt else [])
+        received = self._run(source, dest, flags, shared_server)
+        assert os.path.isfile(os.path.join(received, "keep.txt")), "regular file missing"
+        assert not os.path.lexists(os.path.join(received, "empty_sub")), \
+            f"-a -m created an empty source directory at {received}/empty_sub"
+
+    @pytest.mark.ci
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_collision_at_dir_time_path_does_not_abort(self, shared_server, mt):
+        """P7 Wave D #1: a pre-existing regular file at a source-empty-dir's
+        mirror path must not abort the transfer (the old mkdir failed and failed
+        the run) and must not be clobbered."""
+        source = os.path.join(TEST_DATA_DIR, f"dirtime_collide_{'m' if mt else 's'}_src")
+        dest = os.path.join(TEST_DATA_DIR, f"dirtime_collide_{'m' if mt else 's'}_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        with open(os.path.join(source, "keep.txt"), "wb") as fh:
+            fh.write(b"regular file\n")
+        os.makedirs(os.path.join(source, "collide"))
+        # Plant a regular file at exactly the mirror path of source/collide.
+        received = get_dest_received_dir(dest, source)
+        os.makedirs(received, exist_ok=True)
+        blocker = os.path.join(received, "collide")
+        with open(blocker, "wb") as fh:
+            fh.write(b"pre-existing blocker\n")
+        flags = ["-a"] + (["--threads"] if mt else [])
+        result, _ = run_client(source, dest, flags=flags, port=shared_server.port)
+        assert result.returncode == 0, \
+            f"-a aborted on a pre-existing file at an empty-dir path: " \
+            f"{(result.stderr or result.stdout)[:400]}"
+        assert os.path.isfile(blocker) and not os.path.islink(blocker), \
+            "the pre-existing blocker was replaced by a directory"
+        with open(blocker, "rb") as fh:
+            assert fh.read() == b"pre-existing blocker\n", "the blocker file was clobbered"
+        assert os.path.isfile(os.path.join(received, "keep.txt")), "regular file missing"