✨ feat: support Python 3.15/3.15t and require Python 3.11+

[ShigureNyako]

Changes

• Require Python 3.11+, replace the 3.10 classifier with 3.15, and align Ruff's minimum.
• Update the existing CI matrices: lint on 3.11; test 3.11–3.15 and 3.15t. UV_PYTHON selects the actual matrix interpreter; no added tests or workflows.
• Raise typing-extensions to 4.14.0, the first stable release fixing imports after Python 3.15 removed typing.no_type_check_decorator. Update only that dependency and remove obsolete Python <3.11 backports from the generated lock.

Validation

• 8 tests passed on each of 3.11, 3.12, 3.13, 3.14, 3.15.0rc2 and free-threaded 3.15.0rc2 with locked typing-extensions==4.14.0.
• Dependency imports passed; 3.15t kept the GIL disabled before and after local pytest.
• Python 3.11 lint/format, lock check, sdist/wheel build, actionlint and diff checks passed.

The two-line UV_PYTHON selector is retained because PATH-only local experiments with fresh environments selected 3.14 instead of the requested 3.15/3.15t. README additions, CI hardening and runtime-verification steps have been removed; the net diff is four files, +13/-31.