feat(progress): opt-in paths-only pre-count for rsync to-chk parity

- when --progress/--info=progress is requested, a metadata-only pre-scan
  builds the full file-list total and directory names so the to-chk
  denominator counts every regular/dir/link/special entry like rsync
- per-directory/symlink/special name lines emitted; sequential and --threads
- reuses the --delete-during/delay pre-scan when present; non-progress runs
  take no extra pass
- --progress stays a caveat (emission order still differs); single-file output
  remains byte-identical

tests/integration/test_differential_parity.py

@@ -173,6 +173,12 @@ _CASES = [
            stdout=H.STDOUT_OUTFMT, ref="--out-format %n %l"),
     H.Case("out_format_i_n", "basic", ["-a", "--out-format=%i %n"],
            stdout=H.STDOUT_OUTFMT, ref="--out-format %i %n"),
+    H.Case("progress", "multidir", ["-a", "--progress"], stdout=H.STDOUT_PROGRESS,
+           ci=True, ref="--progress multi-directory file list"),
+    H.Case("progress_threads", "multidir", ["-a", "--progress"],
+           fastsync_flags=["-a", "--progress", "--threads"],
+           stdout=H.STDOUT_PROGRESS, ci=True,
+           ref="--progress multi-directory file list (--threads)"),
 
     # --- transfer modifications -------------------------------------------
     H.Case("update", "basic", ["-a", "--update"], seed=seed_update,