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/unit/test_refined_dfg.py

@@ -28,18 +28,17 @@ def test_source_pin():
     mrt_history = historical_manifest("MRT")
     assert mrt_history["commit"].startswith("3ea131e")
     assert historical_manifest("MRT", "window")["commit"].startswith("a233c90")
-    from source_provenance import HistoryUnavailable, historical_sha256
+    # Negative controls first, so missing history can never hide them: every pinned
+    # mode refuses a working tree that differs from its pin.
+    for collision, mode in (("MRT", "last_base_step"), ("SRT", "last_base_step"), ("MRT", "window")):
+        with unittest.TestCase().assertRaisesRegex(RuntimeError, "Source mismatch"):
+            r.verify_source(r.DEFAULT_SOURCE, collision, mode)
+    from source_provenance import HistoryUnavailable, history_unavailable, verify_manifest
     for manifest in (historical, mrt_history, historical_manifest("MRT", "window")):
         try:  # historical integrity: each manifest matches the tree of its own commit
-            actual = historical_sha256(r.DEFAULT_SOURCE, manifest["commit"], manifest["sha256"])
+            verify_manifest(r.DEFAULT_SOURCE, manifest["commit"], manifest["sha256"])
         except HistoryUnavailable as error:
-            unittest.TestCase().skipTest(f"historical integrity NOT checked: {error}")
-        assert actual == manifest["sha256"], manifest["commit"]
-    # Negative controls: the pinned modes refuse a working tree that differs from their pin.
-    with unittest.TestCase().assertRaisesRegex(RuntimeError, "Source mismatch"):
-        r.verify_source(r.DEFAULT_SOURCE, "MRT")
-    with unittest.TestCase().assertRaisesRegex(RuntimeError, "Source mismatch"):
-        r.verify_source(r.DEFAULT_SOURCE, "SRT")
+            history_unavailable(unittest.TestCase(), error)
 
 
 def test_matched_units_and_planes():