fix(fs): recreate empty dirs, replace blocking file; -R --no-implied-dirs implied parents

- Recursive scans emit a directory entry for every traversed directory that
  produced no transferred child, so empty source dirs (and dirs emptied by
  filtering) are recreated like rsync; -m prunes them, --files-from/--list-only
  never emit implicit dirs.
- A directory entry now replaces a destination regular file (rsync removes the
  non-directory) instead of aborting; confined to the secure parent fd.
- -R --no-implied-dirs --files-from: stop refusing a listed file whose parent
  is not listed; create the implied parent with default attributes (no source
  metadata is captured for it), matching rsync 3.4.1.
- Differential gate: drop min_size/empty_dirs_recursive/dirs_plain allowlist
  entries (dirs_plain now hands FastSync the same trailing-slash source as
  rsync); add differential test for the files-from implied parent.
- Docs: -d row -> Parity, tally 108/24/24.

No wire change (PROTOCOL_VERSION stays 2.26.0).

tests/integration/parity_caveats.py

@@ -25,40 +25,6 @@ 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