test: fix V4 dirty mock, reject in-checkout output, typed blob errors

- test_dfg_re20: the dirty-checkout test now supplies all three git
  responses (HEAD:src, HEAD, status) and asserts the call order, so it
  reaches the clean-checkout rejection instead of StopIteration.
- run_dfg_re20: --output inside the source checkout is refused before
  provenance, plan writing or any CUDA work (receipts would dirty the
  recorded source); tested with the checkout root and nested paths.
- source_provenance.historical_files: validates all paths first and
  reports an unreadable blob as HistoryUnavailable, not a raw
  CalledProcessError; tested with a failing git show.

No physics thresholds changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_validation_source_provenance.py

@@ -98,3 +98,16 @@ def test_historical_check_rejects_tampering_and_reports_missing_history(tmp_path
         verify_manifest(ROOT, ORIGIN, tampered)  # one flipped digest is caught
     with pytest.raises(ValueError, match="not part of"):
         historical_sha256(ROOT, ORIGIN, ["src/CelerisLab/does_not_exist.py"])
+
+
+def test_historical_files_reports_unreadable_blob_as_history_unavailable(tmp_path):
+    """A blob missing from the clone is HistoryUnavailable, not a raw CalledProcessError."""
+    import source_provenance
+    from unittest.mock import patch
+
+    missing = subprocess.CompletedProcess(["git", "show"], 128, b"", b"fatal: missing blob")
+    with patch.object(source_provenance, "historical_sha256", return_value={"src/a.py": "0" * 64}), \
+         patch.object(source_provenance.subprocess, "run", return_value=missing):
+        with pytest.raises(HistoryUnavailable, match="blob not in this clone"):
+            source_provenance.historical_files(ROOT, ORIGIN, ["src/a.py"], tmp_path)
+    assert not (tmp_path / "src/a.py").exists()