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/usage.c

@@ -103,8 +103,9 @@ void print_usage(void) {
   printf("  -R, --relative    With --files-from, preserve each listed entry's relative path\n");
   printf("                    below the destination root instead of mirroring the full\n");
   printf("                    source path (no effect without --files-from)\n");
-  printf("  --no-implied-dirs  With -R --files-from, refuse to place a listed file whose\n");
-  printf("                    parent directory is not itself listed\n");
+  printf("  --no-implied-dirs  With -R, do not apply the source metadata of a listed file's\n");
+  printf("                    implied parent directories (they are still created with\n");
+  printf("                    default attributes)\n");
   printf("  --mkpath          Create the destination root directory on the server when it\n");
   printf("                    does not exist yet\n");
   printf("  --exclude <pattern>, --exclude=<pattern>  Exclude files matching pattern\n");