test(retrieval-gate): red -- row 7's bar is 90 % of a list in the file it guards

PM's J3: seven duplicate `M03 k = 1` mutants appended to `MUTANTS` took row 7
from `11 of 13 RED` to `18 of 20 GREEN` with the SAME two survivors. The bar
is `MUTANT_BAR` of `len(mutants)`, and `MUTANTS` is a module constant in the
file a capability session edits -- a gate whose pass mark is a list the
candidate writes.

Reproduced here at 18 of 20 exactly. 1 red on an assertion about behaviour.

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

tests/test_retrieval_gate.py

@@ -961,3 +961,18 @@ def test_j1_a_registration_this_session_wrote_is_not_a_hold_out(tmp_path: Path)
     # 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
+
+
+def test_j3_row_seven_refuses_a_mutant_list_that_is_not_the_pinned_roster(
+    tmp_path: Path,
+) -> None:
+    """PM's J3: seven duplicate `k = 1` mutants appended to `MUTANTS` took the
+    row from 11 of 13 RED to 18 of 20 GREEN with the SAME two survivors. The
+    bar was 90 % of a list living in the file a capability session edits."""
+    cases, _ = gate.synthetic_cases(tmp_path / "bundles", FIXTURES)
+    baseline = gate.deterministic_rows(cases)
+    padded = (*gate.MUTANTS, *(gate.MUTANTS[2] for _ in range(7)))
+    row = gate.row_seven(cases, baseline, mutants=padded)
+    assert row.status != gate.GREEN, row
+    assert row.fails
+    assert "roster" in row.reason or "duplicate" in row.reason