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/nodes/BlockNode.tsx

@@ -12,6 +12,8 @@ export interface BlockNodeData extends Record<string, unknown> {
   properties: string[];
   /** For constraint kinds, optional one-line expression. */
   expression?: string;
+  /** Highest severity of any validation issue anchored to this block. */
+  issueSeverity?: "error" | "warning" | "soft";
 }
 
 const STEREO: Record<string, string> = {
@@ -30,6 +32,7 @@ export function BlockNode({ data, selected }: NodeProps) {
     "sysml-node",
     `sysml-node-${kind}`,
     selected ? "sysml-node-selected" : "",
+    d.issueSeverity ? `sysml-node-issue-${d.issueSeverity}` : "",
   ]
     .filter(Boolean)
     .join(" ");