fix(scan): attach change relationships in linear time

The third wall under the 13.3 replay, found by the streamed case log: the
evidence and context passes finished at 14:53, then nothing -- no engine
call, no log line -- for 82 minutes, until the case was killed 60 s past
its 7,200 s deadline. `affected_context` tested membership with
`relation not in relationships` on a list: 7,998,000 comparisons for 4,000
relationships (profiled, 83% of the time), and a push over a WordPress
plugin attaches tens of thousands. The deadline is checked once per
question, so a single late question could run for minutes, which is also
why the process overran its cancellation allowance.

Membership now goes through a set beside the ordered list, and each
distinct context path is resolved once instead of once per row. Measured:
4,000 relationships 7.4 s -> 0.44 s; 400,000 in 16 s. The control counts
comparisons rather than timing them, and fails on the old code with
7,998,000.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

tests/test_model_change_context.py

@@ -451,3 +451,24 @@ def test_the_evidence_pass_names_the_changed_files_to_the_engine(tmp_path):
     assert asked and all("contextFilter" not in r for r in asked)
     assert scan._changed_files(context("x\ny.php")) is None
     assert scan._changed_files(context("source.php")) == ("source.php",)
+
+
+def test_attaching_relationships_is_linear_in_what_it_attaches(tmp_path, monkeypatch):
+    """Membership went through a list: 8 million comparisons for 4,000 relationships, and a push over a
+    WordPress plugin ran 82 minutes past its evidence pass and overran the deadline. Counted, not timed, so the
+    control cannot flake under load."""
+    from openultrasast.model.contracts import AffectedRelationship, QuestionIdentity
+    from openultrasast.model.regions import affected_context
+
+    (tmp_path / "source.php").write_text("x\n" * 400)
+    compared = []
+    original = AffectedRelationship.__eq__
+    monkeypatch.setattr(AffectedRelationship, "__eq__", lambda self, other: compared.append(1) or original(self, other))
+    questions = [QuestionIdentity("unit", "php", f"f{i}.php", "run", "injection") for i in range(200)]
+    rows = {
+        q: [{"kind": "context_summary"}] + [{**row("source.php", function=f"m{j}"), "startLine": 1, "endLine": 400} for j in range(20)]
+        for q in questions
+    }
+    enriched, _ = affected_context(context(), questions, rows, tmp_path)
+    assert len(enriched.relationships) == 4000
+    assert len(compared) < 4000, f"{len(compared)} comparisons for 4,000 relationships"