#3672 Flaky: probeStartServer 'a closed port is dead' gets unknown under the parallel test run

src/infrastructure/process/start_instance_probe_test.ts, test 'probeStartServer: a closed port is dead', failed once in verify-build (deno run test, parallel) on 2026-10-20 with Actual unknown / Expected dead. It opens a listener on port 0, closes it, then probes that port. Under the parallel full suite another test can bind the freed port in between, so the probe gets an answer that is not a refusal and reports unknown. In isolation it passed 60/60 (--repeats=20, three runs). Seen on the swamp-club#3637 branch (86b40b5b), which does not touch the probe or its test; the same test passed in the previous verify-build run a few minutes earlier. A fix could hold a port that is known to refuse (for example bind and keep a socket that never accepts, or probe a reserved port) rather than reusing a freed ephemeral one.