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

src/shared/format.h

@@ -88,7 +88,12 @@ typedef struct {
   unsigned long long total_file_size;       /* sum of entry sizes (link target len) */
   unsigned long long transferred_regular;   /* regular files actually stored */
   unsigned long long transferred_file_size; /* source size of those files */
-  unsigned long long literal_data;          /* literal bytes sent for them */
+  /* Whole-file accuracy: the `--stats` "Literal data" row.  The sender counts
+   * the source size of every stored file, so a whole-file transfer matches
+   * rsync.  A delta run actually ships only the literal fragments of the diff
+   * (the rest is matched/copied), so here the value is an upper bound, not
+   * rsync's literal-byte total; see RSYNC_COMPAT.md's `--stats` row. */
+  unsigned long long literal_data;
 } TransferStats;
 
 #endif