✨ feat: require Python 3.11 and test Python 3.15/3.15t

[ShigureNyako]

Changes

• Require Python 3.11; align classifiers, Ruff, lint CI, and the lockfile (remove only obsolete 3.10 backports).
• Add 3.15/3.15t to the existing full test matrix. No source/test changes or extra CI steps.
• Set UV_PYTHON to the matrix version: without it, a fresh uv environment selected 3.14 even with 3.15t first on PATH. This two-line setting is needed to actually test free-threaded Python.

Validation

• Existing 89 tests pass on each of 3.11–3.14, 3.15.0rc2, and 3.15.0rc2 free-threaded; locally verified GIL disabled before/after the 3.15t suite.
• Python 3.11 lint/format and wheel/sdist build pass.
• Final-head CI: six test jobs each pass 89 tests, including genuine 3.15t; lint/format passes.

[SigureMo]

必要性？

[ShigureNyako]

这两行需要保留：实测将 3.15t 放在 PATH 首位、使用全新虚拟环境，不设 UV_PYTHON 时 uv sync 仍选择 3.14；设为 3.15t 才选择 free-threaded 解释器。仅依靠 setup-python 的较小改法会测错版本。其余断言步骤和新增测试已在 4b81d96 删除；当前 CI 确认使用 3.15.0rc2+freethreaded，原有 89 项测试通过。

[SigureMo]

必要性？

[ShigureNyako]

确实不必为此次版本升级新增断言步骤，已在 4b81d96 整段删除，恢复现有测试流程。

[SigureMo]

必要性？

[ShigureNyako]

这些 CLI 测试不是版本升级的必要改动，已在 4b81d96 删除整个新增文件。现有 89 项测试在六个 Python 版本的 CI 上均通过。