Implement client cost attribution

tests/test_coupling.py

@@ -21,4 +21,59 @@ def test_build_service_cost_report_groups_by_service():
     report = build_service_cost_report(lines)
     assert report["signal"] == "service_cost_attribution"
     assert len(report["services"]) == 2
-    assert report["totals_by_month"]["2026-06"] == pytest.approx(99.8)
+    assert report["totals_by_month"]["2026-06"] == pytest.approx(99.8)
+    assert report["attributions"][0]["cost_attribution_key"] is None
+
+
+def test_build_service_cost_report_groups_by_client_attribution():
+    lines = [
+        ServiceCostLine(
+            service_id="cluster",
+            environment="production",
+            period_month="2026-07",
+            amount=42.0,
+            currency="EUR",
+            source="fixture",
+            client_id="acme",
+            application_id="portal",
+            app_instance_id="prod-01",
+            cost_attribution_key="client:acme|app:portal|instance:prod-01",
+        )
+    ]
+
+    report = build_service_cost_report(lines)
+
+    assert report["attributions"] == [
+        {
+            "cost_attribution_key": "client:acme|app:portal|instance:prod-01",
+            "client_id": "acme",
+            "application_id": "portal",
+            "app_instance_id": "prod-01",
+            "currency": "EUR",
+            "months": {"2026-07": 42.0},
+            "total": 42.0,
+        }
+    ]
+
+
+def test_client_attribution_totals_do_not_mix_currencies():
+    common = {
+        "service_id": "cluster",
+        "environment": "production",
+        "period_month": "2026-07",
+        "source": "fixture",
+        "client_id": "acme",
+        "application_id": "portal",
+        "app_instance_id": "prod-01",
+    }
+    report = build_service_cost_report(
+        [
+            ServiceCostLine(amount=42.0, currency="EUR", **common),
+            ServiceCostLine(amount=50.0, currency="USD", **common),
+        ]
+    )
+
+    assert {(row["currency"], row["total"]) for row in report["attributions"]} == {
+        ("EUR", 42.0),
+        ("USD", 50.0),
+    }