test: LES a135252 guard tests use the pinned tree, not today's src

The a135252 manifest is historical qualification: it pins the committed
corrected tree at a135252. The guard is correct; the four tests used the
current working tree as the 'pinned' fixture, so 87c3a31's jets.py change
broke them. Fixtures are now materialized from a135252 with git; the
corrected acceptance, baseline rejection, four tamper rejections and the
reversed-header baseline control are unchanged. A new test records that
the guard refuses today's src where it differs from the pin.

Missing history skips with the reason, or fails when
CELERIS_REQUIRE_HISTORY=1 (release gating) via history_unavailable().

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

tests/validation/source_provenance.py

@@ -48,3 +48,23 @@ def historical_tree(repo, commit, prefix="src"):
     historical_sha256(repo, commit, [])
     return subprocess.check_output(["git", "-C", str(repo), "ls-tree", "-r", "--name-only",
                                     commit, "--", prefix], text=True).splitlines()
+
+
+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, [])
+    for path in paths:
+        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)
+
+
+def history_unavailable(testcase, error):
+    """Skip with the reason, or fail when CELERIS_REQUIRE_HISTORY=1 (release gating)."""
+    import os
+    message = f"historical integrity NOT checked: {error}"
+    if os.environ.get("CELERIS_REQUIRE_HISTORY") == "1":
+        raise AssertionError(message)
+    testcase.skipTest(message)