Establish the interrupt test signal state

[SaladDay]

The SIGINT forwarding test can hang when its runner inherits SIGINT ignored: the process shim correctly preserves that disposition, so the test never delivers a catchable interrupt. Establish a catchable SIGINT in the normal test driver and bound the output wait, with cleanup on every failure path. Production signal behavior and child fixture modes are unchanged.

Validation: the original test deterministically times out under an ignored-SIGINT parent; the fixed interrupt and blocked-output tests pass 50 race repetitions each under both default and ignored parents. The full privileged suite (26 tests), ordinary package tests, vet and repository hygiene checks pass. CI’s original signal disposition was not recorded, so the reproduction establishes a matching fixture defect rather than its unique cause.

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