test(mutants): a surviving mutant is not exit zero (H4, red)

The harness returned `2 if errors else 0`, so a run printing `killed 0
of 1` and naming its survivor exited 0. PM measured it on a copy
carrying only the X2 mutant. Red on the claim: the module imports, and
offers no verdict to ask.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_accounting_gate.py

@@ -887,6 +887,19 @@ def test_a_unit_clean_in_only_one_of_the_two_builds_is_not_clean() -> None:
     assert gate.clean_in_every_run([[clean], [dirty]]) == 0
 
 
+def test_a_surviving_mutant_is_not_exit_zero() -> None:
+    """H4: the mutation harness returned `2 if errors else 0`, so a run that
+    printed `killed 0 of 1` and named the survivor exited 0. PM measured it on
+    a copy carrying only the X2 mutant. A harness nothing can fail is a report,
+    not a gate."""
+    import okf_gate_mutants as mutants
+
+    assert mutants.verdict(survived=[], errors=[]) == 0
+    assert mutants.verdict(survived=["X2"], errors=[]) == 1
+    assert mutants.verdict(survived=[], errors=["not applied"]) == 2
+    assert mutants.verdict(survived=["X2"], errors=["not applied"]) == 2
+
+
 def test_the_gate_exits_one_when_a_row_is_red(monkeypatch: pytest.MonkeyPatch) -> None:
     """M14: nothing checked the real command's exit code, so `main` could
     return 0 over a red table and no test would notice."""