Gate finding BN21 r1 2c (Medium, flaky): the race snippet's own purge() pops
tools.connectors* from sys.modules while sibling threads are mid-import, so
importlib also raises AttributeError ('NoneType' object has no attribute
'__dict__') for tools.connectors.tool. That is a harness artifact, not the
deadlock, yet the old `!= "KeyError"` filter kept it and the fixed tree went
red intermittently (8/160 runs under 8x parallel load).
Filter on the bug class itself. Measured: HEAD 30/30 green serial and 160/160
under the same load; BASE tools/connectors/__init__.py 30/30 red
(_DeadlockError in every run).
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