fix(jobs): validate resume checkpoints before allocation

harness/tests/test_job_runner.py

@@ -233,3 +233,59 @@ def test_run_directories_are_private_and_fsynced_before_atomic_replace(tmp_path,
     first_replace = events.index("replace")
     assert "fsync" in events[:first_replace]
     assert "fsync" in events[first_replace + 1 :]
+
+
+def _tamper_checkpoint(tmp_path, report, transform):
+    path = tmp_path / ".tht-jobs" / report.job_type / "runs" / report.run_id / "checkpoint.json"
+    payload = json.loads(path.read_text())
+    transform(payload)
+    path.write_text(json.dumps(payload))
+
+
+@pytest.mark.parametrize(
+    "transform",
+    [
+        lambda payload: payload["stages"].pop(),
+        lambda payload: payload["stages"].reverse(),
+        lambda payload: payload["stages"].append(dict(payload["stages"][0])),
+        lambda payload: payload["stages"][0].update(name="substitute"),
+        lambda payload: payload.update(input_fingerprint="sha256:" + "f" * 64),
+        lambda payload: payload["stages"][0].update(status="pending"),
+    ],
+)
+def test_semantically_tampered_checkpoint_fails_without_orphan_run(tmp_path, transform):
+    def one(_context):
+        pass
+
+    def two(_context):
+        pass
+
+    spec = _spec(tmp_path, stage_ids=("one", "two"))
+    first = run_job(spec, [one, two])
+    runs = tmp_path / ".tht-jobs" / "evidence" / "runs"
+    before = {path.name for path in runs.iterdir()}
+    _tamper_checkpoint(tmp_path, first, transform)
+    called = False
+
+    def forbidden(_context):
+        nonlocal called
+        called = True
+
+    with pytest.raises(CorruptCheckpointError, match="invalid|incompatible"):
+        run_job(spec.with_resume(first.run_id), [forbidden, forbidden])
+    assert called is False
+    assert {path.name for path in runs.iterdir()} == before
+
+
+def test_stored_compatibility_fingerprint_tamper_fails_without_orphan(tmp_path):
+    first = run_job(_spec(tmp_path), [lambda _context: None])
+    runs = tmp_path / ".tht-jobs" / "evidence" / "runs"
+    before = {path.name for path in runs.iterdir()}
+    _tamper_checkpoint(
+        tmp_path,
+        first,
+        lambda payload: payload.update(compatibility_fingerprint="sha256:" + "0" * 64),
+    )
+    with pytest.raises(CorruptCheckpointError, match="invalid"):
+        run_job(_spec(tmp_path, resume_run_id=first.run_id), [lambda _context: None])
+    assert {path.name for path in runs.iterdir()} == before