feat(canvas): implement dropped connection resolution and enhance connection handling

tests/canvas-connection-policy.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  getCanvasConnectionValidationMessage,
+  validateCanvasConnectionPolicy,
+} from "@/lib/canvas-connection-policy";
+
+describe("canvas connection policy", () => {
+  it("limits compare nodes to two incoming connections", () => {
+    expect(
+      validateCanvasConnectionPolicy({
+        sourceType: "image",
+        targetType: "compare",
+        targetIncomingCount: 2,
+      }),
+    ).toBe("compare-incoming-limit");
+  });
+
+  it("describes the compare incoming limit", () => {
+    expect(
+      getCanvasConnectionValidationMessage("compare-incoming-limit"),
+    ).toBe("Compare-Nodes erlauben genau zwei eingehende Verbindungen.");
+  });
+});