test(retrieval-gate): red -- row 5's hold-out is an assertion the file makes about itself

PM's J1: two files written by the session under test came back `7 of 7 GREEN`.
Every one of row 5's checks reads a field the registration owns. "Written by
another session than the ranking change" is `bool()` of a string the file
sets; "no reading predates the threshold" reads a `readings` list from the
same file. The order's point -- report-only without a pre-written threshold is
not a protection -- is not met by a protection that is itself a claim.

1 red on an assertion about behaviour (GREEN for a registration this test
wrote seconds earlier), and it names what a session cannot write about
itself: that git already carried the file before the ranking moved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_retrieval_gate.py

@@ -883,3 +883,33 @@ def test_a_forced_fixture_that_stops_missing_counts_against_row_two(
     row = gate.row_two([case])
     assert (row.k, row.m, row.status) == (0, 1, gate.RED)
     assert any("Z9" in detail and "premise" in detail for detail in row.details)
+
+
+def test_j1_a_registration_this_session_wrote_is_not_a_hold_out(tmp_path: Path) -> None:
+    """PM's J1: two files written by the session under test came back
+    `7 of 7 GREEN`. Every check was an assertion the registration made about
+    itself -- `written_by` is `bool()` of a string the file sets, and the
+    readings are read from a list the same file owns."""
+    held = tmp_path / "held-out.json"
+    held.write_text('{"questions": []}', encoding="utf-8")
+    registration = tmp_path / "registration.json"
+    registration.write_text(
+        json.dumps(
+            {
+                "set": str(held),
+                "sha256": gate.sha256_of(held),
+                "threshold": "hit@8 must not fall below 6 of 8",
+                "threshold_written_at": "2026-01-01",
+                "written_by": "the same session, lying",
+                "readings": [],
+            }
+        ),
+        encoding="utf-8",
+    )
+    row = gate.row_five(registration)
+    assert row.status != gate.GREEN, row
+    assert row.fails
+    # What a session cannot write about itself: that git already carried the
+    # file before the ranking moved.
+    assert any("committed" in detail for detail in row.details), row.details
+    assert any("ranking" in detail for detail in row.details), row.details