Add Python 3.15 and 3.15t support

[XXXXRT666]

• ☑️ I understand it is strictly prohibited to use AI to write PR description
• 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.

[zcbenz]

Running test_bindings.py with debug build DEBUG=1 setup.py build_ext -i gets critical error:

Critical nanobind error: attempted to change the reference count of a Python object or to call into Python while the GIL was not held!

[zcbenz]

There is also a weird error when building wheel https://github.com/ml-explore/mlx/actions/runs/36394870740/job/108838778494:

  /usr/bin/ccache /usr/bin/c++ -DNB_DOMAIN=mlx -Dcore_EXPORTS -I/home/runner/work/_temp/uv-python-dir/cpython-3.15.0rc2-linux-x86_64-gnu/include/python3.15 -I/home/runner/work/mlx/mlx/build/temp.linux-x86_64-cpython-315/mlx.core/_deps/nanobind-src/include -I/home/runner/work/mlx/mlx -O3 -DNDEBUG -std=gnu++20 -flto=auto -fno-fat-lto-objects -fPIC -fvisibility=hidden -fno-stack-protector -ffunction-sections -fdata-sections -Werror -MD -MT python/src/CMakeFiles/core.dir/device.cpp.o -MF python/src/CMakeFiles/core.dir/device.cpp.o.d -o python/src/CMakeFiles/core.dir/device.cpp.o -c /home/runner/work/mlx/mlx/python/src/device.cpp
  In file included from /home/runner/work/_temp/uv-python-dir/cpython-3.15.0rc2-linux-x86_64-gnu/include/python3.15/Python.h:14,
                   from _deps/nanobind-src/include/nanobind/nb_python.h:21,
                   from _deps/nanobind-src/include/nanobind/nanobind.h:32,
                   from /home/runner/work/mlx/mlx/python/src/device.cpp:6:
  /home/runner/work/_temp/uv-python-dir/cpython-3.15.0rc2-linux-x86_64-gnu/include/python3.15/pyconfig.h:2067: error: "_POSIX_C_SOURCE" redefined [-Werror]
   2067 | #define _POSIX_C_SOURCE 202405L
        | 
  In file included from /usr/include/x86_64-linux-gnu/c++/11/bits/os_defines.h:39,
                   from /usr/include/x86_64-linux-gnu/c++/11/bits/c++config.h:586,
                   from /usr/include/c++/11/utility:68,
                   from /usr/include/c++/11/optional:36,
                   from /home/runner/work/mlx/mlx/python/src/device.cpp:3:
  /usr/include/features.h:290: note: this is the location of the previous definition
    290 | # define _POSIX_C_SOURCE        200809L
        | 

[XXXXRT666]

structure_sentinel() still stored an owning static nb::object, which performed a DECREF after shutting down

Some translation units included standard headers before Python.h. This caused glibc to define some macros before Python 3.15's pyconfig.h defined them with different values

This 2 problems are all fixed now

[zcbenz]

Thanks!