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).

src/client/scanner.h

@@ -151,6 +151,11 @@ typedef struct {
   bool capture_dir_times;
   ArrayList* dir_entries;
   mtx_t* dir_entries_mutex;
+  /* --no-implied-dirs with -R + --files-from: a directory that is only an
+   * implied parent of a listed entry (not itself listed, nor below a listed
+   * directory) must not carry source metadata; it is created with default
+   * attributes at the destination, matching rsync. */
+  bool no_implied_dirs;
 } ScannerOptions;
 
 /* Internal per-scanner filter state. FilterNode chains represent the ordered
@@ -168,6 +173,11 @@ typedef struct {
   int current_depth;
   dev_t root_dev;
   bool failed;
+  /* Recursive scan: whether the open directory yielded any transferred or
+     descended entry.  When it did not, closing it emits a directory entry so
+     the empty source directory is recreated at the destination (rsync
+     parity). */
+  bool current_dir_produced;
   /* Phase 2 (files-from / filter layer). */
   char* root_path;          /* transfer root (fs path) for rel computation */
   char* current_rel;        /* rel path of the open directory ("" == root) */