gh-152330: Make perf_trampoline thread safe.

[YukaSadaoka]

Summary of Changes

Fixes multiple critical data races, post-fork deadlocks, memory leaks, and edge-case exception handling issues in perf_trampoline.c under Free Threading and Python 3.14.

• Cold-Path Mutex (perf_trampoline_mutex): Introduced a dedicated mutex to guard global state mutations and JIT arena allocations (compile_trampoline(), initialization, and teardown). Hot-path executions remain lock-free.
• Atomic State Management: Converted reads and writes for perf_status, extra_code_index, and persist_after_fork to atomic operations.
• Safe Teardown Fallback: Updated py_trampoline_evaluator to load extra_code_index atomically. If deactivation has started (returning -1), evaluating threads cleanly bypass the JIT trampoline and fall back to _PyEval_EvalFrameDefault instead of hitting assertion failures.
• Non-Blocking I/O: Decoupled map file writing (write_state writing to /tmp/perf-PID.map) from the critical section, ensuring slow disk I/O does not hold the compiler lock.
• Deadlock Prevention: Added a post-fork child handler to force-reset perf_trampoline_mutex to an unlocked state, preventing deadlocks if a thread forks while another thread holds the lock.
• Graceful Child Teardown: If trampoline re-initialization fails post-fork in a child process, the exception is cleared (PyErr_Clear()) and the trampoline is disabled gracefully rather than leaking unhandled exceptions or crashing.
• Watcher ID Sentinel Fix: Fixed a sentinel collision in pycore_ceval_state.h by initializing the global watcher ID sentinel to -1 instead of 0. This allows 0 to be correctly recognized as a valid active watcher ID.

Fixes gh-152330

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The following commit authors need to sign the Contributor License Agreement:

• yukasadaoka@google.com

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