nx_hotlisten_gate.nx

nx_hotlisten_gate.nx -- R5 proof: SO_REUSEPORT is what makes a ZERO-DOWNTIME handoff possible. license_tier: ORIGINAL THE CLASS UNDER TEST: today every /api/deploy and /api/restart drops the in-flight request (FETCH-FAIL, ~10x in one session) because the daemon is KILLED while answering, and we wrote DOCTRINE around that ("503 = EXPECTED, do NOT retry-hammer") -- a doctrine patch over a missing mechanism. A second process cannot take the port until the first is gone, so there is always a gap. SO_REUSEPORT removes the gap. T1 is the NON-VACUITY CONTROL and it is the whole point: it proves the SECOND bind genuinely FAILS on the standard listener. Without T1, T2 succeeding would prove nothing -- it could just mean the port was free. A cure with no proven disease is not a proof.