fix(mutants): a surviving mutant exits 1 (H4)

`verdict(survived, errors)` is the one place the run's exit code is
decided: 2 when a mutant could not be applied (it was never measured,
and that outranks everything), 1 when one survived, 0 otherwise.

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

tools/okf_gate_mutants.py

@@ -18,6 +18,10 @@ edited, so an interrupted run cannot leave a mutant behind. A pattern that does
 not match the expected number of times is reported as an ERROR and the run
 exits 2 -- a mutant that could not be applied was never measured, and counting
 it as killed is the same mistake as reading an empty search as an absence.
+
+A SURVIVOR EXITS 1. Until 2026-09-19 the run ended `2 if errors else 0`, so
+`killed 0 of 1` with the survivor printed beside it was an exit 0 and nothing
+could fail on the finding this harness exists to produce (H4).
 """
 
 from __future__ import annotations
@@ -26,6 +30,7 @@ import shutil
 import subprocess
 import sys
 import tempfile
+from collections.abc import Sequence
 from dataclasses import dataclass
 from pathlib import Path
 
@@ -267,6 +272,20 @@ def _apply(text: str, mutant: Mutant) -> str:
     return text.replace(mutant.old, mutant.new, 1 if mutant.first_only else -1)
 
 
+def verdict(survived: Sequence[str], errors: Sequence[str]) -> int:
+    """The run's exit code: 0 clean, 1 a mutant survived, 2 one was not measured.
+
+    Until 2026-09-19 this was `2 if errors else 0`, so a run that printed
+    `killed 0 of 1` and named its survivor exited 0 and no caller could fail
+    on it (H4). A mutant that survived names a check nothing holds, which is
+    the finding this harness exists to produce; a mutant that could not be
+    applied was never measured at all, and that outranks it.
+    """
+    if errors:
+        return 2
+    return 1 if survived else 0
+
+
 def main(argv: list[str] | None = None) -> int:
     del argv
     survived: list[str] = []
@@ -311,7 +330,7 @@ def main(argv: list[str] | None = None) -> int:
         print(f"  survived: {label}")
     for problem in errors:
         print(f"  ERROR: {problem}")
-    return 2 if errors else 0
+    return verdict(survived, errors)
 
 
 if __name__ == "__main__":