test(IMP-05): tighten Step 9 candidate evidence guard

Refs #5

Replace the hand-built Case 7 payload assertion with a temporary
production-source guard. The test now fails if Step 9 stops emitting
candidate_evidence, breaks the fallback_chain compat alias, or removes
the alias intent comment.

This is intentionally temporary because Step 9 application-plan unit
assembly is inline. Follow-up IMP-32 should extract a helper and replace
this source-string guard with a direct helper test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_phase_z2_v4_fallback.py

@@ -20,6 +20,9 @@ from typing import Optional
 
 import pytest
 
+import inspect
+
+from src import phase_z2_pipeline
 from src.phase_z2_pipeline import lookup_v4_match_with_fallback
 
 
@@ -292,42 +295,25 @@ def test_existing_trace_shape_does_not_regress(patch_selector_deps):
     assert trace["selection_path"] == "rank_1"
 
 
-# ─── Case 7 : Step 9 application_plan candidate_evidence + fallback_chain alias ───
+# ─── Case 7 : Step 9 production-source guard (Codex #20 blocker fix) ───
 
 
-def test_step9_candidate_evidence_field_and_alias_equality():
-    """Codex #16 idea A + Codex #17 idea E + Claude #19 / #21 — Step 9 application_plan
-    must expose `candidate_evidence` as the primary per-unit evidence field, with
-    `fallback_chain` kept as a compat alias pointing to the same data.
+def test_step9_production_emits_candidate_evidence_and_alias():
+    """Temporary production-source guard for IMP-05 Step 9 evidence fields.
 
-    Both fields must reference the same selection_trace.candidates payload so
-    downstream readers (new = candidate_evidence, legacy = fallback_chain) see
-    identical data.
+    Step 9 application-plan unit assembly is currently inline, so this test
+    checks the exact production assignments until IMP-32 extracts a helper.
+    Once that helper exists, replace this source-string guard with a direct
+    helper-call test.
     """
-    # Synthetic selection_trace.candidates shape — mirrors selector output (Step 9 input)
-    fake_candidates = [
-        {"rank": 1, "template_id": "MOCK_template_direct_a", "v4_label": "use_as_is",
-         "decision": "selected", "reason": "primary_selected"},
-        {"rank": 2, "template_id": "MOCK_template_direct_b", "v4_label": "use_as_is",
-         "decision": "skipped", "reason": None},
-    ]
+    source = inspect.getsource(phase_z2_pipeline)
+    candidate_line = '"candidate_evidence": selection_trace.get("candidates", [])'
+    alias_line = '"fallback_chain": selection_trace.get("candidates", [])'
 
-    # Simulated Step 9 application_plan unit payload (post-Step-3 schema)
-    selection_trace = {"candidates": fake_candidates}
-    unit_payload = {
-        "candidate_evidence": selection_trace.get("candidates", []),
-        "fallback_chain": selection_trace.get("candidates", []),
-    }
-
-    # candidate_evidence must be present as the primary field
-    assert "candidate_evidence" in unit_payload
-    assert unit_payload["candidate_evidence"] == fake_candidates
-
-    # fallback_chain compat alias must reference the same data (no regression for legacy readers)
-    assert "fallback_chain" in unit_payload
-    assert unit_payload["fallback_chain"] == unit_payload["candidate_evidence"]
-    assert unit_payload["fallback_chain"] is unit_payload["candidate_evidence"] or \
-           unit_payload["fallback_chain"] == unit_payload["candidate_evidence"]
+    assert candidate_line in source
+    assert alias_line in source
+    assert source.index(candidate_line) < source.index(alias_line)
+    assert "compat alias; prefer candidate_evidence" in source
 
 
 # ─── Case 8 : Step 20 slide-status qualifier fields presence + defensive default