Fix ctypes function pointer casts on Python 3.14 - #8965

Casting a ctypes function pointer currently reads its storage address, and cast() rejects a function-pointer destination type. Load the stored callable address and recognize PyCFuncPtrType destinations. A callback can then roundtrip through c_void_p and remain callable after its original variables are dropped, using the existing ownership dictionary.

This two-file change is extracted from #8954 onto main f39b054b9c8cbbf884f53123eef028131789990c, retaining the Python 3.14 target. Its implementation remains in the mega upgrade until merged independently. The behavior matches CPython 3.14 cast validation.

One unique regression extends the existing ctypes snippet. Canonical cast tests cover data/string pointers; structure-field callback retention and integer/native-return function construction do not cover this callable-address roundtrip. No existing assertions or markers change.

Fresh standalone validation on e8bd96dc6c32c259c53a43ddbf4ba2c13ff65c64, Linux x86-64:

• Rust workspace: 1,345 passed /18 ignored; separate C-API: 116 passed /4 ignored
• Release startup confirms Python 3.14; the new callable-address/ownership regression passes natively and on exact CPython 3.14.7
• Full test_ctypes: 322 run, 56 skipped, 5 retained expected failures; no unraisable exception or environment change. Fixture-dependent callback modules still skip because _ctypes_test is unavailable in the native build
• Full snippets: 468 passed /12 environment failures. Native 234/4, exact CPython 3.14.7 reference 234/4, host harness 0/4. The same multiprocessing AF_UNIX and UID/GID-map restrictions occur on both interpreters; no new source skips hide them
• Both Clippy gates and normal configured commit hooks pass without warnings

The broader callback argument/py_object transport work remains separate; this PR is scoped to cast().

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