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/config.h

@@ -82,7 +82,7 @@ typedef struct {
 typedef enum SuperMode { SUPER_MODE_AUTO = 0, SUPER_MODE_ON = 1, SUPER_MODE_OFF = 2 } SuperMode;
 
 /* ===========================================================================
- * Config wire-field table (single source of truth for protocol 2.26.0).
+ * Config wire-field table (single source of truth for protocol 2.27.0).
  *
  * Every field below crosses the wire.  The table is the ONLY place a
  * serialized field is named: config.h expands CONFIG_WIRE_FIELDS() to declare