perf(m49): defer fresh stage revalidation

tests/test_m49_portable_executor_release.py

@@ -540,6 +540,32 @@ def test_dynamic_source_materializer_is_source_derived_and_tamper_evident(
         validate_m49_portable_source_stage(stage.root)
 
 
+def test_fresh_source_stage_defers_full_validation_until_execution_boundary(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    original = source_module.validate_m49_portable_source_stage
+    validated_roots: list[Path] = []
+
+    def counting_validator(root: Path) -> source_module.M49PortableSourceStage:
+        validated_roots.append(root)
+        return original(root)
+
+    monkeypatch.setattr(
+        source_module,
+        "validate_m49_portable_source_stage",
+        counting_validator,
+    )
+
+    first_root, _expected = _materialized_source(tmp_path, monkeypatch)
+    assert validated_roots == []
+    validated = original(first_root)
+    assert validated.timeline_frame_count == 3
+
+    second_root, _expected = _materialized_source(tmp_path, monkeypatch)
+    assert second_root == first_root
+    assert validated_roots == [first_root]
+
+
 def test_source_materializer_rejects_unadmitted_adjacent_metadata(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: