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_override_dep_convention.py

@@ -87,7 +87,8 @@ def test_a_request_that_raises_leaves_nothing_it_did_not_commit(db):
 def test_the_commit_check_sees_a_core_dml_write(db, make_user):
     """`db.execute(update(...))` flushes nothing and stages nothing, so the session's own bookkeeping
     never sees it: a handler whose only write is Core DML and that forgot its commit passed. The
-    database's transaction id is what tells. A committed one must still pass."""
+    database's transaction id is what tells — also when the test itself had a write open before the
+    request, which the handler's write would otherwise share. A committed one must still pass."""
     import pytest
     from sqlalchemy import update
     from starlette.requests import HTTPConnection
@@ -96,7 +97,9 @@ def test_the_commit_check_sees_a_core_dml_write(db, make_user):
 
     user = make_user()
 
-    def drive(commit: bool) -> None:
+    def drive(commit: bool, *, test_wrote: bool = False) -> None:
+        if test_wrote:  # the test's own write, open and uncommitted, as the request starts
+            db.execute(update(User).where(User.id == user.id).values(display_name="test"))
         conn = HTTPConnection(
             {"type": "http", "method": "POST", "path": "/probe", "headers": [], "query_string": b""}
         )
@@ -113,3 +116,8 @@ def test_the_commit_check_sees_a_core_dml_write(db, make_user):
     drive(commit=False)
     assert db.info["unfinished"] == ["POST /probe"]
     db.info["unfinished"].clear()  # recorded as it should be — not this test's failure
+    drive(commit=True, test_wrote=True)
+    assert db.info["unfinished"] == []
+    drive(commit=False, test_wrote=True)
+    assert db.info["unfinished"] == ["POST /probe"]
+    db.info["unfinished"].clear()