Use ASCII case folding for HTML element and attribute names

[ac1982]

HTMLTranslator currently Unicode-lowercases element and attribute names. Given <x-Ä id="upper"></x-Ä><x-ä id="lower"></x-ä>, selecting x-Ä incorrectly returns lower. [DATA-Ä] likewise selects a data-ä attribute instead of data-Ä.

Use the existing ascii_lower helper for HTML name normalization, consistent with HTML/CSS ASCII case-matching rules. ASCII portions still fold (X-Ä matches x-Ä), while non-ASCII characters retain their identity. XML/XHTML and the attribute-value extension point are unchanged.

Two regressions fail against master and pass with the fix; they assert the generated XPath directly, covering ordinary, mixed-ASCII-case, and wildcard-namespace selectors. The original lxml-parsed HTML reproductions were also rerun separately against the revised tree and passed. This is unrelated to #50 (Python 2 byte-string input), #189/#191 (tokenization), and the recent attribute-value i flag support.

Validation on Python 3.14.4 / Linux arm64:

• Review revision: 29 tests/docs passed, mypy, pylint and all pre-commit hooks passed; both simplified regressions fail against unchanged master.
• Original submission checks (builds not rerun for this test-only revision):
• Full tests including documentation examples: 29 passed; XPath module 100% coverage, total 99%.
• Ruff lint/format, mypy (5 source files), pylint, complete pre-commit hooks, Sphinx build with -W, sdist build / twine check, and git diff --check passed.
• Other Python versions, PyPy, macOS and Windows were not run locally.

AI assistance: OpenAI Codex investigated, implemented, and ran these checks.

[AdrianAtZyte]

Could you simplify the tests to assert on the generated XPath, like most tests in that file do? e.g. assert xpath("X-Ä") == "descendant-or-self::x-Ä".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (805705c) to head (fa9244e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #200   +/-   ##
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  Coverage   99.79%   99.79%           
=======================================
  Files           3        3           
  Lines         987      987           
  Branches      161      161           
=======================================
  Hits          985      985           
  Misses          1        1           
  Partials        1        1           

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