Add Python 3.14/3.14t support and Cython 3.3

[jlevy]

I have some downstream projects that use cydifflib and it would be great if I can support 3.14 properly.
Rest of the description and code changes are agent written.
I added a little modernization with uv and an AGENTS.md but tried to keep changes otherwise minimal.
—Josh

Summary

Already in place and unchanged:

• 3.9+ drop-in for stdlib difflib, with no runtime dependencies
• scikit-build-core + CMake
• sdist without Cython after generate + tools/sdist.patch
• pip/cibuildwheel as the published path

This PR adds 3.14 and free-threaded 3.14t support and the Cython / concurrency work those builds require.

Problem

• No 3.14 or free-threaded 3.14t wheels.
• Cython was >=3.0.12,<3.1.0. 3.14t does not compile on Cython 3.0.x.
• No # cython: freethreading_compatible=True, so a 3.14t import re-enables the GIL.
• No documented or tested concurrency contract. SequenceMatcher scratch is per-instance; sharing one matcher across threads is unsupported (same as stdlib). HtmlDiff._default_prefix is process-wide.
• Local uv workflow was not set up (optional; not a user-facing break).

Changes

• Cython >=3.3.0,<3.4 (build-only); scikit-build-core >=1.0; CMake max 3.30.
• freethreading_compatible=True pragma; HtmlDiff._default_prefix lock; docs; tests/test_concurrency.py.
• Wheel matrix includes 3.14/3.14t with cibuildwheel 4.2.1 and small portability fixes (3.9 zip without strict=, skip thread tests when threads cannot start, Windows ARM64 delvewheel exclude MSVC CRT); Intel Mac CI uses macos-15-intel because macos-13 runners are retired.
• Optional local uv (uv.toml / uv.lock); no .python-version; AGENTS.md is contributor docs only.
• The sdist job installs the Cython-stripped tarball and runs pytest.

Compatibility

• Python 3.9+ and the public API are unchanged.
• Optional uv does not change pip, conda, thefuzz, PyPy, or sdist-without-Cython. CI stays on pip.

Dependencies

• Runtime: none.
• Build: Cython and scikit-build-core bumps (cmake/ninja stay out of build-system.requires).
• Dev: optional pytest>=8.

Test plan

• Local pytest on 3.10, 3.11, 3.12, 3.13, 3.14, and 3.14t.
• Isolated wheel build.
• CI matrix including 3.14/3.14t.

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