fix(core): satisfy strict signature test typing

packages/core/test/signatures.test.ts

@@ -13,7 +13,7 @@ describe('协议签名原语', () => {
     const signature = h(fixture.signature);
     expect(await verifyStatement(publicKey, fixture.kind, payload, signature)).toBe(true);
     expect(await verifyStatement(publicKey, 'chat.edit', payload, signature)).toBe(false);
-    payload[0] ^= 1;
+    payload[0] = payload[0]! ^ 1;
     expect(await verifyStatement(publicKey, fixture.kind, payload, signature)).toBe(false);
     expect(await verifyStatement(publicKey, fixture.kind, h(fixture.payload), signature.subarray(1))).toBe(false);
     expect(fromBase64Url(fixture.x)).toHaveLength(32);