feat(canvas): finalize mixer reconnect swap and related updates

tests/canvas-connection-validation.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+
+import { validateCanvasEdgeSplit } from "@/components/canvas/canvas-connection-validation";
+
+describe("validateCanvasEdgeSplit", () => {
+  it("uses middle-node target handle for first split leg", () => {
+    const reason = validateCanvasEdgeSplit({
+      nodes: [
+        { id: "node-source", type: "image", position: { x: 0, y: 0 }, data: {} },
+        { id: "node-target", type: "compare", position: { x: 400, y: 0 }, data: {} },
+      ],
+      edges: [
+        {
+          id: "edge-source-target",
+          source: "node-source",
+          target: "node-target",
+          targetHandle: "left",
+        },
+      ],
+      splitEdge: {
+        id: "edge-source-target",
+        source: "node-source",
+        target: "node-target",
+        targetHandle: "left",
+      },
+      middleNode: { id: "node-middle", type: "mixer", position: { x: 200, y: 0 }, data: {} },
+    });
+
+    expect(reason).toBeNull();
+  });
+});