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/lib/sysml/model.ts

@@ -0,0 +1,75 @@
+// Canonical SysML metamodel types — adapted from phase-0/src/types.ts
+// (the Phase 0 validation that ran 10/10 against the corpus).
+//
+// See docs/sysml-modeling.md §6 for the design rationale and §3 for what we
+// deliberately don't model.
+
+export type BlockKind = "block" | "actor" | "constraint" | "system";
+
+export type PropertyType =
+  | { kind: "string" }
+  | { kind: "number" }
+  | { kind: "boolean" }
+  | { kind: "enum"; values: string[] };
+
+export type Multiplicity = "0..1" | "1" | "0..*" | "1..*";
+
+export interface Property {
+  id: string;
+  name: string;
+  type: PropertyType;
+  multiplicity: Multiplicity;
+  description?: string;
+}
+
+export interface Block {
+  id: string;
+  label: string;
+  kind: BlockKind;
+  stereotypes: string[];
+  properties: Property[];
+  description?: string;
+}
+
+export type AssociationKind =
+  | "association"
+  | "composition"
+  | "aggregation"
+  | "generalization"
+  | "constraintApplies";
+
+export interface Association {
+  id: string;
+  fromBlockId: string;
+  toBlockId: string;
+  label: string;
+  kind: AssociationKind;
+  multiplicity?: { from: Multiplicity; to: Multiplicity };
+}
+
+export interface Constraint {
+  id: string;
+  label: string;
+  expression: string;
+  appliesTo: string[];
+}
+
+export type RequirementRelation =
+  | { kind: "satisfy"; blockId: string }
+  | { kind: "derive"; fromReqId: string }
+  | { kind: "verify"; experimentId: string };
+
+export interface Requirement {
+  id: string;
+  tag: string;
+  text: string;
+  relations: RequirementRelation[];
+}
+
+export interface SysMLModel {
+  systemOfInterestId?: string;
+  blocks: Block[];
+  associations: Association[];
+  constraints: Constraint[];
+  requirements: Requirement[];
+}