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/validation/source_provenance.py

@@ -66,12 +66,15 @@ def historical_tree(repo, commit, prefix="src"):
 def historical_files(repo, commit, paths, dest):
     """Write each path as stored in `commit` under `dest` (HistoryUnavailable if missing)."""
     from pathlib import Path
-    historical_sha256(repo, commit, [])
+    historical_sha256(repo, commit, paths)  # HistoryUnavailable / ValueError before writing
     for path in paths:
+        shown = subprocess.run(["git", "-C", str(repo), "show", f"{commit}:{path}"],
+                               capture_output=True)
+        if shown.returncode:
+            raise HistoryUnavailable(f"cannot read {commit}:{path} (blob not in this clone)")
         target = Path(dest) / path
         target.parent.mkdir(parents=True, exist_ok=True)
-        target.write_bytes(subprocess.run(["git", "-C", str(repo), "show", f"{commit}:{path}"],
-                                          check=True, capture_output=True).stdout)
+        target.write_bytes(shown.stdout)
 
 
 def history_unavailable(testcase, error):