fix(builder): correct move and sortable behavior

packages/blocks/test/engine.test.ts

@@ -137,4 +137,36 @@ describe("dynamic blocks engine", () => {
     expect(decoded.ok && decoded.value.roots[0]?.id).not.toBe(node.id);
     expect(duplicateNodes([node], id)[0]?.props).toEqual(node.props);
   });
+
+  test("moves forward within one slot without an off-by-one error", () => {
+    const document = doc();
+    document.root.slots.content = ["a", "b", "c"].map((nodeId) => ({
+      ...text.create({ id: () => nodeId }),
+      props: { text: nodeId },
+    }));
+    const moved = applyCommand(document, {
+      type: "move",
+      intent: {
+        nodeIds: ["a"],
+        from: [{ parentId: "root", slot: "content", index: 0 }],
+        to: { parentId: "root", slot: "content", index: 3 },
+      },
+    }, kind, registry);
+    expect(moved.ok && moved.value.document.root.slots.content.map((node) => node.id)).toEqual([
+      "b", "c", "a",
+    ]);
+  });
+
+  test("duplicates blocks with deterministic injected ids", () => {
+    const document = doc();
+    document.root.slots.content = [{ ...text.create({ id: () => "a" }), props: { text: "A" } }];
+    const duplicated = applyCommand(document, {
+      type: "duplicate",
+      nodeIds: ["a"],
+      ids: ["a-copy"],
+    }, kind, registry);
+    expect(duplicated.ok && duplicated.value.document.root.slots.content.map((node) => node.id)).toEqual([
+      "a", "a-copy",
+    ]);
+  });
 });