refactor(arg_tokenizer): dedupe double-dash-flag predicate; derive seen_dash_dash

- has_double_dash_flag/double_dash_flag_value/double_dash_flag_values
  shared the same 3-part match condition inline three times; factored
  into is_double_dash_flag(). double_dash_flag_value keeps its own
  find() (not double_dash_flag_values().next()) since that would subtly
  change semantics: "first match regardless of whether it has a value"
  vs "first match that has a value".
- seen_dash_dash was always exactly emitted_dash_dash && dialect ==
  Posix (only ever set true immediately after emitted_dash_dash inside
  the Posix branch); removed the redundant variable and derived the
  condition directly. Pure refactor, no behavior change either way.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>