fix: derive complete trace blocks from private artifact

tests/test_materialize_workload_regimes.py

@@ -1,4 +1,5 @@
 import importlib.util
+import json
 import sys
 from pathlib import Path
 
@@ -82,3 +83,22 @@ def test_family_axes_change_only_the_declared_contract():
         "prompt-1",
         "prompt-2",
     ]
+
+
+def test_private_source_projects_only_complete_prefix_blocks(tmp_path):
+    private_path = tmp_path / "source.jsonl"
+    source = {
+        "source_index": 7,
+        "arrived_at": 1.5,
+        "input_length": 17,
+        "output_length": 4,
+        "session_id": 11,
+        "runtime_block_ids": [101, 102],
+        "body": {"prompt": "fixture"},
+    }
+    private_path.write_text(json.dumps(source) + "\n")
+
+    rows = MODULE.load_source(None, private_path)
+
+    assert rows[0]["runtime_block_ids"] == [101]
+    assert rows[0]["source_arrived_at"] == 1.5