feat: implement Stories 2.4-2.7 — E-R, Org Chart, Architecture, Sequence diagram type renderers

Adds four diagram type renderers completing the core diagram type suite:
- Story 2.4: E-R entity nodes with column tables, relationship edges with cardinality labels
- Story 2.5: Org chart person nodes with role/department tags, hierarchy edges
- Story 2.6: Architecture nodes (service, database, queue, load balancer, external), connection edges
- Story 2.7: Sequence participant nodes with lifelines + activation bars, fragment nodes,
  3 custom edge types (sync/async/return), custom time-ordered layout (not ELK)

Story 2.7 includes code review fixes: computeLayout returns LayoutResult so enriched
sequence edges flow through useAutoLayout, activation bar computation in layout,
immutable layout function, self-message U-shaped loop rendering, sequence node size
tests in buildElkGraph.

476 tests passing across 29 test files, zero regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/modules/diagram/hooks/useAutoLayout.ts

@@ -11,7 +11,7 @@ import {
   SOFT_CAP_NODE_COUNT,
 } from "../lib/elk-layout";
 
-import type { ElkLayoutOptions } from "../lib/elk-layout";
+import type { ElkLayoutOptions, LayoutResult } from "../lib/elk-layout";
 
 const DEBOUNCE_MS = 300;
 const LAYOUT_ANIMATION_MS = 200;
@@ -24,6 +24,7 @@ export function useAutoLayout() {
 
   const nodeCount = useGraphStore((s) => s.nodeCount);
   const setNodes = useGraphStore((s) => s.setNodes);
+  const setEdges = useGraphStore((s) => s.setEdges);
   const layoutDirection = useGraphStore((s) => s.layoutDirection);
   const edgeRouting = useGraphStore((s) => s.edgeRouting);
   const isLayouting = useGraphStore((s) => s.isLayouting);
@@ -53,7 +54,7 @@ export function useAutoLayout() {
       flowNodes.forEach((el) => el.classList.add("layouting"));
 
       try {
-        const layoutedNodes = await computeLayout(
+        const result: LayoutResult = await computeLayout(
           currentNodes,
           currentEdges,
           {
@@ -67,7 +68,10 @@ export function useAutoLayout() {
           },
         );
 
-        setNodes(layoutedNodes);
+        setNodes(result.nodes);
+        if (result.edges) {
+          setEdges(result.edges);
+        }
 
         // Fit view after layout, with a small delay to let transition run
         setTimeout(() => {
@@ -96,7 +100,7 @@ export function useAutoLayout() {
         }, LAYOUT_ANIMATION_MS);
       }
     },
-    [setNodes, setIsLayouting, fitView],
+    [setNodes, setEdges, setIsLayouting, fitView],
   );
 
   const triggerLayout = useCallback(