v1.6.1

1.6.1: ordered options, aligned help, version spacing

Three fixes to usage/help composition:

- options are listed in their declared order. Since 1.5.0 `options` was a
  std::unordered_map (to drop the tsl::ordered_map dependency) with a "reverse
  iterate to keep order" routine, but unordered_map has no insertion order, so
  help listed options in hash order. options is now an ordered
  std::vector<std::pair<std::string, option_t>>.
- the description column is aligned by measuring the widest rendered option
  first, then padding. The previous up-front width miscounted the key-column
  padding, so long options ran straight into their descriptions.
- the program name is no longer followed by a stray space when version_title
  starts on a new line (title()/version()).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>