feat(three-stage-scan): reject unsafe regression snippets before execution

src/openultrasast/regress/candidate.py

@@ -0,0 +1,41 @@
+"""Safety-gated regression execution (task 4.3)."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from ..sandbox import SandboxJob, SandboxResult, SandboxRunner
+from .safety import UnsafeSnippetError, check_snippet_safety
+
+INCONCLUSIVE = "inconclusive"
+SAFETY_REJECTED = "safety_rejected"
+TRIGGERABLE = "triggerable"
+NOT_TRIGGERABLE = "not_triggerable"
+
+
+@dataclass(frozen=True)
+class RegressionVerdict:
+    verdict: str
+    reason: str = ""
+
+
+class RegressionRunner:
+    """Run a snippet in the sandbox only after the structural safety check passes."""
+
+    def __init__(self, sandbox: SandboxRunner) -> None:
+        self._sandbox = sandbox
+
+    def run_snippet(self, snippet: str, job: SandboxJob) -> RegressionVerdict:
+        try:
+            check_snippet_safety(snippet)
+        except UnsafeSnippetError:
+            return RegressionVerdict(verdict=INCONCLUSIVE, reason=SAFETY_REJECTED)
+        return _verdict_from_result(self._sandbox.run(job))
+
+
+def _verdict_from_result(result: SandboxResult) -> RegressionVerdict:
+    if result.timed_out:
+        return RegressionVerdict(verdict=INCONCLUSIVE, reason="timeout")
+    if result.exit_code != 0:
+        return RegressionVerdict(verdict=TRIGGERABLE, reason="nonzero_exit")
+    return RegressionVerdict(verdict=NOT_TRIGGERABLE, reason="exit_zero")