fix(investigation): persist large results and require graph links

Full-network pressure comparisons exceeded the worker limit before saved-result previews could run. Preserve complete calculation results and keep read-time summaries and pagination.

Incremental graph publication accepted isolated nodes after flow-authoring rules were lost. Require new steps to connect explicitly while retaining historical snapshots and status-only patches. Regression tests reproduce both failures on the previous code.

tests/unit/test_investigation_graph.py

@@ -239,6 +239,85 @@ def test_graph_renderer_escapes_labels_and_keeps_explicit_parallel_and_failed_pa
     assert graph.node_refs[2].status == "reference"
 
 
+@pytest.mark.parametrize("edges", [[], [{"source": "check", "target": "check"}]])
+def test_new_multistep_graph_requires_relationships_instead_of_isolated_nodes(edges):
+    with pytest.raises(DomainError) as error:
+        update_investigation_graph(None, InvestigationGraphUpdate.model_validate({
+            "nodes": [{"node_id": "read", "label": "观测核验"},
+                      {"node_id": "check", "label": "异常判断"}],
+            "edges": edges,
+        }))
+    assert error.value.code == "GRAPH_RELATION_REQUIRED"
+    assert "check" in str(error.value)
+
+
+def test_new_branch_must_connect_to_the_saved_narrative_without_inventing_edges():
+    original = update_investigation_graph(None, InvestigationGraphUpdate.model_validate({
+        "nodes": [{"node_id": "root", "label": "调查起点"}],
+    }))
+    with pytest.raises(DomainError) as error:
+        update_investigation_graph(original, InvestigationGraphUpdate.model_validate({
+            "nodes": [{"node_id": "left", "label": "独立核查"},
+                      {"node_id": "right", "label": "核查发现"}],
+            "edges": [{"source": "left", "target": "right"}],
+        }))
+    assert error.value.code == "GRAPH_RELATION_REQUIRED"
+    assert original.edges == []
+    assert len(original.node_refs) == 1
+
+
+def test_connected_parallel_branches_and_join_render_top_down():
+    graph = update_investigation_graph(None, InvestigationGraphUpdate.model_validate({
+        "nodes": [{"node_id": key, "label": label} for key, label in (
+            ("start", "异常核验"), ("isolation", "隔离分析"),
+            ("impact", "爆管影响"), ("summary", "汇总判断"))],
+        "edges": [{"source": source, "target": target, "label": label}
+                  for source, target, label in (
+                      ("start", "isolation", "并行核查"), ("start", "impact", "并行核查"),
+                      ("isolation", "summary", "汇总依据"), ("impact", "summary", "汇总依据"))],
+    }))
+    assert graph.mermaid.startswith("flowchart TD\n")
+    assert len(graph.edges) == 4
+    assert 'start -->|"并行核查"| impact' in graph.mermaid
+    patched = update_investigation_graph(graph, InvestigationGraphUpdate.model_validate({
+        "nodes": [{"node_id": "impact", "status": "recorded"}],
+    }))
+    assert patched.edges == graph.edges
+
+
+def test_existing_disconnected_snapshot_remains_readable_and_patchable():
+    original = InvestigationDiagram.model_validate({
+        "title": "已保存的旧图", "mermaid": 'flowchart TD\n  read["观测"]\n  check["判断"]',
+        "edges": [], "node_refs": [{"node_id": "read", "label": "观测"},
+                                   {"node_id": "check", "label": "判断"}],
+    })
+    patched = update_investigation_graph(original, InvestigationGraphUpdate.model_validate({
+        "nodes": [{"node_id": "check", "status": "recorded"}],
+    }))
+    assert patched.mermaid == original.mermaid
+    assert patched.edges == original.edges == []
+    assert original.node_refs[1].status is None
+
+
+@pytest.mark.anyio
+async def test_publication_rejects_missing_links_and_accepts_a_corrected_graph():
+    task, repo, service = setup_service()
+    first = await publish(service, repo, task["task_id"], nodes=[
+        {"node_id": "read", "label": "观测核验"},
+    ])
+    original = first.task.latest_progress.progress.diagram.model_dump(mode="json")
+    with pytest.raises(DomainError) as error:
+        await publish(service, repo, task["task_id"], nodes=[
+            {"node_id": "check", "label": "异常判断"},
+        ])
+    assert error.value.code == "GRAPH_RELATION_REQUIRED"
+    assert (await service.detail(task["task_id"])).latest_progress.progress.diagram.model_dump(mode="json") == original
+    corrected = await publish(service, repo, task["task_id"], nodes=[
+        {"node_id": "check", "label": "异常判断"},
+    ], edges=[{"source": "read", "target": "check", "label": "核验依据"}])
+    assert 'read -->|"核验依据"| check' in corrected.task.latest_progress.progress.diagram.mermaid
+
+
 def test_first_structured_update_preserves_an_existing_mermaid_without_reinterpreting_it():
     original = InvestigationDiagram.model_validate({
         "title": "原调查路线", "mermaid": 'flowchart LR\nA{核查条件} -->|具备| B[原路径]\nA --> C[另一分支]',