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Fix free-threaded data race in the C LookupBase cache (#380) (#382)

* Fix free-threaded data race in the C LookupBase cache (#380)

On a free-threaded (no-GIL) build, concurrent lookup() and changed() calls
on a LookupBase could crash the interpreter (PyMutex_Unlock / SIGSEGV): the
_cache/_mcache/_scache fields were created (ASSURE_DICT) and cleared
(Py_CLEAR) without synchronization. The earlier PyDict_GetItemRef migration
fixed the dict-contents borrowed references but not the field lifecycle.

Guard the field lifecycle with a critical section on Python 3.13+ (a no-op
block earlier, where the GIL serializes): the create/read publishes a dict
allocated outside the lock and returns a strong reference under the lock, and
changed() detaches the fields under the lock and decrefs them after releasing
it. No Python runs while the lock is held, so it cannot deadlock. This
guarantees memory safety; it does not make invalidation linearizable (a
lookup may still populate a retired cache generation), which matches the
cache's semantics.

Adds a subprocess-isolated concurrency regression test.

Assisted-by: Claude Code:claude-fable-5-1

* Fix remaining LookupBase cache race window

* Fix VerifyingBase concurrent invalidation

* Reuse LB_changed() in verify_changed() and document last-writer-wins publish

- verify_changed() now invalidates the lookup caches through LB_changed()
  instead of detaching the three LB fields by hand, so the cache fields are
  enumerated in LB_traverse, LB_clear and LB_changed only. Invalidation
  becomes two state transitions; the comment records that a concurrent
  reader may observe cleared caches with the previous verification pair
  still installed and verifies against that pair as before.
- The publish of the _verify_ro/_verify_generations pair is last-writer-wins;
  the comment now says so and states the redundant invalidation as the
  accepted cost.

No test change. The concurrency module and the full suite pass with this
chan... (continued)

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