flaky perry-stdlib test: thread_exit_releases_the_threads_external_buffer_registrations ('EXTERNAL_BUFFER_REGISTRY outlived the thread', ~1/2000) #11547

runtime_thread_exit_tests::symbols_tests::thread_exit_releases_the_threads_external_buffer_registrations failed once in about 2,000 filtered runs of runtime_thread_exit_tests on main. The runs were on perrymaster with CI env (CARGO_PROFILE_TEST_DEBUG=0 CARGO_PROFILE_DEV_DEBUG=0 CARGO_INCREMENTAL=0). Assertion:

EXTERNAL_BUFFER_REGISTRY outlived the thread

Likely cause, unverified: the test checks membership by address only, the same defect #11544 found for SYMBOL_POINTERS (#11539). A sibling test's buffer can reuse the freed block and register the same address. Suggested fix: check by identity (address plus a monotonically increasing id or generation), as #11544 does. Keep a mutation check to show the test still catches a real release gap.