The `static thread_local std::vector<PyThreadContextEntry>` holds
`nb::object` references to Python Context/Location/InsertionPoint
objects. When a Context is entered (pushed onto the stack) but never
exited before interpreter shutdown, the thread-local storage destructor
runs after `Py_Finalize()` on the main thread, attempting `Py_DECREF`
through the dead runtime → SIGSEGV.
Fix: Register an `atexit` handler that clears the stack while the
interpreter is still alive, releasing all held Python references before
finalization.
Maksim Levental
ead7fbd791