fix(preprocess): harden crash recovery integrity

harness/tests/test_corpus_publish.py

@@ -41,3 +41,18 @@ def test_active_pointer_cannot_escape_generation_root(tmp_path):
     store.active_path.write_text("../outside\n")
     with pytest.raises(UnsafeCorpusPath):
         store.active_manifest()
+
+
+def test_publish_restores_previous_active_when_directory_fsync_fails_after_replace(tmp_path, monkeypatch):
+    store = CorpusStore(tmp_path / "corpus")
+    first = store.stage(CorpusManifest(), {})
+    second = store.stage(CorpusManifest(), {})
+    store.publish(first)
+    def fail_once():
+        store._fsync_directory = store._sync_root
+        raise OSError("post replace crash")
+
+    store._fsync_directory = fail_once
+    with pytest.raises(OSError, match="post replace"):
+        store.publish(second)
+    assert store.active_generation() == first