Fix file-variable CLI flag and header ID setup

[ryanchou1994]

--use-file-vars now works as a flag. Previously argparse expected a value, so placing it before an input filename consumed the filename; using it with stdin or after a filename failed with a missing-argument error.

This also fixes a crash when an Emacs-style file variable enables header-ids. Those options are loaded after construction, so they need the same normalization as constructor extras before heading rendering. The shared helper preserves initialization behavior and existing reset-count settings.

Related to #365 and #269. This addresses the CLI flag and header-id file variables, rather than every file-variable extra. The README includes a working command and modeline example.

Compatibility: the flag no longer accepts a value. A token after --use-file-vars is an input path; --use-file-vars=VALUE is rejected.

Validation:

• Regression tests failed on the base: five CLI failures and one API error, with the no-flag control passing.
• Mac Python 3.13.13: make testone passed 275 tests; the seven existing ReDoS regressions passed.
• Linux Python 3.13.15: the same 275 functional tests, seven existing ReDoS tests, internal doctests, and 12 installed-wheel API/CLI checks passed. All 887 source file hashes matched before and after validation.
• New CLI tests pass pycodestyle. The two existing changed Python files retain the same 475 pre-existing pycodestyle diagnostics, with none added.
• Wheel and source distributions build; Twine passes with the existing missing long_description_content_type warning.

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