fix(growth): stabilize comparison coverage cohort

tests/test_pchome_revenue_growth_service.py

@@ -330,6 +330,45 @@ def test_pchome_growth_coverage_states_are_mutually_exclusive():
     assert stats["comparison_metric_contract"]["state_partition"]["mutually_exclusive"] is True
 
 
+def test_pchome_growth_coverage_scope_is_fixed_before_mapping_priority():
+    from services.pchome_revenue_growth_service import build_pchome_growth_opportunities
+
+    engine = create_engine("sqlite:///:memory:")
+    _seed_growth_tables(engine)
+    with engine.begin() as conn:
+        conn.execute(text("""
+            INSERT INTO daily_sales_snapshot
+                ("商品ID", "商品名稱", snapshot_date, "總業績", "數量", "商品館")
+            VALUES
+                ('PCH-3', '營收第三名', '2026-06-14', 150000, 30, '保養'),
+                ('PCH-4', '營收第四名', '2026-06-14', 145000, 29, '保養'),
+                ('PCH-5', '營收第五名', '2026-06-14', 140000, 28, '保養'),
+                ('PCH-6', '營收第六名', '2026-06-14', 135000, 27, '保養')
+        """))
+
+    payload = build_pchome_growth_opportunities(engine, limit=5)
+    product_ids = {
+        item["pchome_product_id"] for item in payload["opportunities"]
+    }
+    contract = payload["stats"]["comparison_metric_contract"]
+
+    assert "PCH-1" in product_ids
+    assert "PCH-2" not in product_ids
+    assert contract["version"] == "pchome_comparison_coverage_v3"
+    assert contract["scope"] == "top_5_revenue_products"
+    assert contract["numerator"] == 1
+    assert contract["denominator"] == 5
+    assert contract["rate"] == 20.0
+    assert contract["scope_membership"] == {
+        "selection_stage": "before_action_priority_scoring",
+        "ranking": "max_current_or_previous_7d_revenue_desc_then_qty_desc_then_product_id_asc",
+        "member_count": 5,
+        "fingerprint_sha256": contract["scope_membership"]["fingerprint_sha256"],
+        "mapping_status_can_change_membership": False,
+    }
+    assert len(contract["scope_membership"]["fingerprint_sha256"]) == 24
+
+
 def test_momo_review_candidates_return_dual_store_links_and_plain_reasons():
     from services.external_market_offer_service import list_momo_review_candidates