MVP M4: SysML metamodel + validator + dependency graph + UI surfacing

apps/web/lib/sysml — pure engine (no UI deps)
- model.ts: canonical types ported from phase-0 (Block, Property,
  Association, Constraint, Requirement, SysMLModel)
- validate.ts: 12 rules grouped Structural (S1–S5) / Semantic (M1–M5) /
  Traceability (T1–T2). Pure function returning element-anchored issues.
  Cycle detection via DFS with canonical-key dedup. T3 deferred to Phase 1.5
  (depends on narrative chip references).
- depgraph.ts: directed graph over Blocks / Properties / Constraints /
  Requirements with `dependentsOf()` and `neighborhood()` for impact analysis
  (substrate for M7).
- fromFixture.ts: loose FixtureData → strict SysMLModel converter (interim
  until M5 unifies state).
- breaks.ts: 8 named model corruptions (S1/S2/S3/M2/M3/M5/T1/T2) for
  ?break=... demonstrations.

UI integration in EditorShell + LeftRail + DiagramCanvas + IssuesPanel
- Floating IssuesPanel (bottom-right) with severity-grouped counts and
  collapse. Click an issue to focus its anchor across the rail and diagram.
- LeftRail: severity dots on each block in the Model section and on each
  requirement entry. Tooltips show full issue text.
- DiagramCanvas: red/blue/dotted ring around offending blocks via
  issuesByElement → BlockNode.data.issueSeverity.
- ?break=S1-dangling,M2-cycle,M5-dup-property,... query param injects
  corruptions for verification of the "done when" criterion.
- Suspense boundary added at the editor route for useSearchParams.

Two pre-existing TipTap typing fixes (editor.storage cast through unknown).

apps/web/components/diagram-canvas/DiagramCanvas.tsx

@@ -35,6 +35,7 @@ import { NodeInspector } from "./NodeInspector";
 import { fixtureToFlow } from "./fixtureToFlow";
 import type { Density } from "../socrates/SocratesDock";
 import type { FixtureData, AssociationKind, BlockKind } from "../../lib/fixtures/aristotle";
+import type { ValidationIssue } from "../../lib/sysml/validate";
 
 export type DiagramVariant = "softened" | "formal" | "graph";
 
@@ -44,6 +45,7 @@ interface DiagramCanvasProps {
   variant?: DiagramVariant;
   focusBlockId: string | null;
   onSelect?: (id: string | null) => void;
+  issuesByElement?: Map<string, ValidationIssue[]>;
 }
 
 const nodeTypes = { sysmlBlock: BlockNode };
@@ -54,6 +56,13 @@ let nextEdgeId = 1000;
 function freshNodeId(): string { return `n${nextNodeId++}`; }
 function freshEdgeId(): string { return `e${nextEdgeId++}`; }
 
+function pickWorst(issues: ValidationIssue[]): "error" | "warning" | "soft" | undefined {
+  if (issues.some(i => i.severity === "error")) return "error";
+  if (issues.some(i => i.severity === "warning")) return "warning";
+  if (issues.some(i => i.severity === "soft")) return "soft";
+  return undefined;
+}
+
 export function DiagramCanvas(props: DiagramCanvasProps) {
   return (
     <ReactFlowProvider>
@@ -62,7 +71,7 @@ export function DiagramCanvas(props: DiagramCanvasProps) {
   );
 }
 
-function DiagramInner({ data, focusBlockId, onSelect }: DiagramCanvasProps) {
+function DiagramInner({ data, focusBlockId, onSelect, issuesByElement }: DiagramCanvasProps) {
   const initial = useMemo(() => fixtureToFlow(data), [data]);
   const [nodes, setNodes] = useState<Node<BlockNodeData>[]>(initial.nodes);
   const [edges, setEdges] = useState<Edge<SysmlEdgeData>[]>(initial.edges);
@@ -90,14 +99,20 @@ function DiagramInner({ data, focusBlockId, onSelect }: DiagramCanvasProps) {
   }, []);
 
   // Highlight focusBlockId from sibling components (e.g. narrative chip hover).
+  // Also surface validation severity onto each node so BlockNode can ring it.
   useEffect(() => {
     setNodes(ns =>
-      ns.map(n => ({
-        ...n,
-        selected: n.id === focusBlockId,
-      }))
+      ns.map(n => {
+        const issues = issuesByElement?.get(n.id) ?? [];
+        const severity = pickWorst(issues);
+        return {
+          ...n,
+          selected: n.id === focusBlockId,
+          data: { ...(n.data as BlockNodeData), issueSeverity: severity },
+        };
+      })
     );
-  }, [focusBlockId]);
+  }, [focusBlockId, issuesByElement]);
 
   const onNodeClick: NodeMouseHandler = useCallback(
     (_event, node) => {