fix(vector): default missing tangents to {x:0, y:0} in create_vector

Vectors created via MCP with straight-line segments (no tangent
handles) crashed the .fig serializer on save. Segments without
tangentStart/tangentEnd are now normalized to zero vectors at
creation time, producing straight line segments as intended.

Fixes #206

packages/core/src/index.ts

@@ -26,6 +26,7 @@ export {
   SceneGraph,
   generateId,
   cloneVectorNetwork,
+  normalizeVectorNetwork,
   type SceneNode,
   type NodeType,
   type Fill,

packages/core/src/scene-graph/index.ts

@@ -78,6 +78,25 @@ export function cloneVectorNetwork(vn: VectorNetwork): VectorNetwork {
   }
 }
 
+/**
+ * Ensure every segment has tangentStart/tangentEnd.
+ * Missing tangents default to {x:0, y:0} (straight line segments).
+ * Use at system boundaries where input may come from JSON/MCP.
+ */
+export function normalizeVectorNetwork(vn: VectorNetwork): VectorNetwork {
+  const ZERO: Vector = { x: 0, y: 0 }
+  return {
+    vertices: vn.vertices,
+    segments: vn.segments.map((s) => ({
+      start: s.start,
+      end: s.end,
+      tangentStart: (s as Partial<VectorSegment>).tangentStart ?? { ...ZERO },
+      tangentEnd: (s as Partial<VectorSegment>).tangentEnd ?? { ...ZERO }
+    })),
+    regions: vn.regions
+  }
+}
+
 export interface GeometryPath {
   windingRule: WindingRule
   commandsBlob: Uint8Array

packages/core/src/tools/create.ts

@@ -1,6 +1,7 @@
 import { parseColor } from '../color'
 import { fetchIcons, searchIconsBatch } from '../icons'
 import { createIconFromPaths } from '../icons/render'
+import { normalizeVectorNetwork } from '../scene-graph'
 import { defineTool, nodeSummary } from './schema'
 
 import type { FigmaNodeProxy } from '../figma-api'
@@ -169,7 +170,8 @@ export const createVector = defineTool({
     node.y = args.y
     if (args.name) node.name = args.name
     if (args.path) {
-      figma.graph.updateNode(node.id, { vectorNetwork: JSON.parse(args.path) as VectorNetwork })
+      const network = normalizeVectorNetwork(JSON.parse(args.path) as VectorNetwork)
+      figma.graph.updateNode(node.id, { vectorNetwork: network })
     }
     if (args.fill) {
       node.fills = [{ type: 'SOLID', color: parseColor(args.fill), opacity: 1, visible: true }]