test: tighten historical-pin checks per review of 941eb28b

- test_refined_dfg::test_source_pin runs its mismatch negative controls
  first (now including window mode) so missing history cannot skip them.
- test_re100: renamed to test_manifest_matches_its_pinned_commit, plus an
  explicit record that today's src differs from the pin.
- Structural manifest checks run before any history-dependent check.
- Real tamper test: a flipped digest raises via verify_manifest, and a
  path absent at the commit raises ValueError.
- A blobless clone (tree present, blob missing) is reported as unavailable
  history, not as a missing path.
- All history skips go through history_unavailable(): fatal when
  CELERIS_REQUIRE_HISTORY=1.

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

tests/validation/run_jet_circle_cv.py

@@ -223,12 +223,11 @@ def verify(repo):
 
 def verify_historical(repo):
     """The pinned manifest describes the PIN tree (read with git, not the working tree)."""
-    from source_provenance import historical_sha256
+    from source_provenance import verify_manifest
     manifest = json.loads((HERE / "data/jet_circle_cv/source_manifest.json").read_text())
     if manifest["revision"] != PIN:
         raise RuntimeError("Wrong pinned revision")
-    if historical_sha256(repo, PIN, manifest["files"]) != manifest["files"]:
-        raise RuntimeError("Historical manifest does not match its pinned revision")
+    verify_manifest(repo, PIN, manifest["files"])
     return manifest