Fix case-insensitive stable sorting of response headers

[xu91102]

Use a lowercase field-name sort key when formatting headers. Previously repeated fields with different capitalization could be reordered, despite the formatter's order-preservation contract.

Python's stable sort now groups case-insensitively equal names while preserving their original order, spelling and values. The status line is unchanged. A regression includes differently capitalized Set-Cookie fields and neighboring fields; it fails before the fix.

Validation: all 122 output tests pass (3 expected failures), plus Flake8 and diff checks. Tests ran with proxy environment variables removed so the invalid-host test exercises its intended network error. Prepared with Codex assistance.

Fixes #1960

Remote CI baseline note: two Big5 charset-detection tests fail in an untouched checkout of base 5b604c3; this change does not modify charset detection.