test: check historical source manifests against their recorded commits

Manifests and receipts unchanged. Each of the 8 group-A identity tests now
extracts its manifest's recorded commit and verifies hashes there instead of
against the current checkout. Tamper assertions and runner source refusals are
kept; missing recorded history is an explicit failure, not a skip or repin.

tests/unit/historical_tree.py

@@ -0,0 +1,46 @@
+"""Check historical source manifests against their own recorded commit, not today's checkout."""
+
+import io
+from pathlib import Path
+import subprocess
+import tarfile
+
+ROOT = Path(__file__).resolve().parents[2]
+
+
+def _require(commit):
+    """Missing recorded history is an explicit failure, never a skip or a repin."""
+    found = subprocess.run(
+        ["git", "-C", str(ROOT), "cat-file", "-e", commit + "^{commit}"],
+        capture_output=True,
+    )
+    if found.returncode:
+        raise AssertionError(
+            f"Required history missing: commit {commit} is not in this clone; "
+            "fetch it instead of repinning or substituting"
+        )
+
+
+def tracked(commit, prefix="src"):
+    """Tracked paths under prefix at the recorded commit."""
+    _require(commit)
+    return set(
+        subprocess.check_output(
+            ["git", "-C", str(ROOT), "ls-tree", "-r", "--name-only", commit, prefix],
+            text=True,
+        ).splitlines()
+    )
+
+
+def historical_tree(commit, dest):
+    """Extract the tracked tree of the recorded commit into dest."""
+    _require(commit)
+    archive = subprocess.check_output(
+        ["git", "-C", str(ROOT), "archive", "--format=tar", commit]
+    )
+    dest = Path(dest)
+    dest.mkdir(parents=True, exist_ok=True)
+    extra = {"filter": "data"} if hasattr(tarfile, "data_filter") else {}
+    with tarfile.open(fileobj=io.BytesIO(archive)) as tar:
+        tar.extractall(dest, **extra)
+    return dest