Make harness stop conservative for ablation

src/aituner/harness.py

@@ -452,6 +452,7 @@ def _convergence_guard(
     ):
         reason = "strong_incumbent_requires_validation_probes"
     return {
+        "deterministic_stop": should_stop,
         "should_stop_if_no_harness_can_justify_a_new_adjacent_probe": (
             should_stop
             or bool(infeasible_progress["stop_if_next_probe_repeats_family"])
@@ -474,7 +475,7 @@ def _harness_stop_decision(
     recent_diagnostics: list[dict[str, Any]],
 ) -> dict[str, Any]:
     guard = _convergence_guard(state, recent_diagnostics)
-    if guard["should_stop_if_no_harness_can_justify_a_new_adjacent_probe"]:
+    if guard["deterministic_stop"]:
         return {
             "should_stop": True,
             "reason": guard["reason"],