test: the commit check starts every request clean, and the fence docs claim what it holds

The test's own writes are committed before a request, so a handler's Core DML can no longer hide
in a transaction the test had already opened. The entry-point probe answers on a marked line the
API's logging cannot mix into. A listed commit whose function no longer commits fails the fence.
CLAUDE.md and the backend README now name what an AST cannot see; the route-commit tests no longer
claim their read-back holds the commit.

backend/tests/test_unit_fences.py

@@ -636,6 +636,27 @@ def test_every_unit_package_exists():
             assert (APP / entry.split(":")[0]).exists(), entry
 
 
+def test_every_listed_commit_is_still_there():
+    """An entry names a commit that exists: one left behind by a rename, or by a function that no
+    longer commits, would go on allowing a commit to whatever takes its name next."""
+    stale = []
+    for commits in (*UNIT_COMMITS.values(), CORE_COMMITS):
+        for entry in commits:
+            rel, _, name = entry.partition(":")
+            tree = ast.parse((APP / rel).read_text(encoding="utf-8"))
+            functions = _enclosing_functions(tree)
+            committing = {
+                functions.get(node)
+                for node in ast.walk(tree)
+                if isinstance(node, ast.Call)
+                and isinstance(node.func, ast.Attribute)
+                and node.func.attr == "commit"
+            }
+            if (name not in committing) if name else not committing:
+                stale.append(entry)
+    assert stale == []
+
+
 PACKAGE_CASES = {
     # (module path, source): violations
     ("downloads/x.py", "from app.models import Video"): 0,  # capture reads catalog