refactor(parity): address code-quality review findings

- cli: remove UB in --bwlimit scaling (range-check the double product before
  casting, drop atoi for the +/-1 form) and add huge/boundary unit tests
- test: widen the CI throttle wall-clock band to [1.5, 4.5]s with a 2s
  cross-tolerance so a loaded runner cannot flake it
- log: drop the unused LOG_INFO_BACKUP bit; --info=backup is accepted-but-
  silent like the other rsync-only categories
- client_send: remove the duplicate delete_display_path forward declaration
- utils: add non-allocating utils_strip_transfer_root and use it from
  scanner_note_nonreg and delete_display_path (was duplicated logic)
- scanner: lstat() instead of stat() when re-reading an empty dir's metadata
- file: drop the no-op else-if and the redundant ELOOP arm in
  file_ensure_directory_secure (symlinks are refused anyway)
- format/stats: document literal_data as whole-file accurate (delta upper
  bound) instead of claiming literal bytes sent
- docs: refresh stale protocol 2.26.0 labels to 2.27.0

tests/integration/test_option_parity.py

@@ -92,9 +92,13 @@ class TestBwlimitParity:
         result, fast_secs = run_client(source, dest, flags=["-a", "--bwlimit=2048"],
                                        port=shared_server.port)
         assert result.returncode == 0, (result.stderr or result.stdout)[:200]
-        assert fast_secs > 1.0, f"fastsync throttled too little: {fast_secs:.2f}s"
-        # Both rendezvous near 2 s; allow a generous band for CI scheduling.
-        assert abs(fast_secs - rsync_secs) < 1.0, (
+        # 4 MiB at 2 MiB/s rendezvous near 2 s.  Use a coarse band on each side
+        # (an unthrottled transfer finishes well under 1.5 s) plus a generous
+        # cross-tolerance so a loaded CI runner cannot flake the parity assert.
+        lo, hi = 1.5, 4.5
+        assert lo <= fast_secs <= hi, f"fastsync throttle out of band: {fast_secs:.2f}s"
+        assert lo <= rsync_secs <= hi, f"rsync throttle out of band: {rsync_secs:.2f}s"
+        assert abs(fast_secs - rsync_secs) < 2.0, (
             f"fastsync {fast_secs:.2f}s vs rsync {rsync_secs:.2f}s"
         )