Add Python 3.15 and 3.15t support

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• AI usage disclosure: All tests are generated by AI, and AI helps me fix bugs

Add Python 3.15 and 3.15t to the release wheel matrix. Update nanobind from 3.0.1 to 3.1.0 and align the extension build configuration and documentation with free-threaded support.

Freeze bound types on Python 3.15 to enable Python immutable type optimizations. This prevents runtime changes to attributes on those types.

Tests on Python 3.15t also exposed two lifetime issues:

• Transform callbacks were stored in nanobind function objects, which are immortal in free threaded builds. The wrapper now owns the C++ callback directly through a gc nanobind class instance, allowing captured objects and compiled function caches to be released.
• The random state sentinel could access the Python runtime during static destruction and crash at shutdown when first initialized on a worker thread. Keep its existing process lifetime reference without a reference count operation during static destruction.