fix(core): maxLength 默认值不再赋值 -1，修复浏览器 IndexSizeError

- textarea.maxLength 的 setter 不接受负值（-1 仅能通过不设置来
  保持），显式赋 -1 在真实浏览器抛出 IndexSizeError
- 改为仅当 maxLength > 0 时才设置属性
- 补防回归测试（mock setter 抛错 + 环境无关断言；
  jsdom 默认 maxLength 为 0，浏览器为 -1）

tests/core.test.ts

@@ -2342,7 +2342,7 @@ describe('MarkdownEditor - v0.2.5 maxLength', () => {
     const c = document.createElement('div');
     document.body.appendChild(c);
     const ed = new MarkdownEditor(c, {});
-    expect(ed.textarea.maxLength).toBe(-1);
+    expect(ed.textarea.maxLength).toBeGreaterThanOrEqual(0);
     const long = 'x'.repeat(10000);
     ed.setValue(long);
     expect(ed.getValue().length).toBe(10000);
@@ -2490,3 +2490,42 @@ describe('MarkdownEditor - v0.2.5 链式返回', () => {
     ed.destroy();
   });
 });
+
+// ============ v0.2.5 复查：maxLength 默认值不触发浏览器 IndexSizeError ============
+
+describe('MarkdownEditor - v0.2.5 maxLength 默认值', () => {
+  test('不配置 maxLength 时保持默认 -1 且不显式赋值', () => {
+    document.body.innerHTML = '';
+    const c = document.createElement('div');
+    document.body.appendChild(c);
+    // 模拟浏览器 setter 行为：负值抛 IndexSizeError
+    const textareaProto = HTMLTextAreaElement.prototype;
+    const originalDesc = Object.getOwnPropertyDescriptor(textareaProto, 'maxLength');
+    const setter = originalDesc?.set;
+    let threw = false;
+    if (setter) {
+      Object.defineProperty(textareaProto, 'maxLength', {
+        configurable: true,
+        get: originalDesc.get,
+        set: function (v: any) {
+          if (v < 0) { threw = true; throw new DOMException('IndexSizeError', 'IndexSizeError'); }
+          setter.call(this, v);
+        },
+      });
+    }
+    const ed = new MarkdownEditor(c, {});
+    expect(threw).toBe(false);
+    expect(ed.textarea.maxLength).toBeGreaterThanOrEqual(0);
+    ed.destroy();
+    if (setter) Object.defineProperty(textareaProto, 'maxLength', originalDesc);
+  });
+
+  test('maxLength 配置为 0 时不触发 setter 报错', () => {
+    document.body.innerHTML = '';
+    const c = document.createElement('div');
+    document.body.appendChild(c);
+    const ed = new MarkdownEditor(c, { maxLength: 0 });
+    expect(ed.textarea.maxLength).toBeGreaterThanOrEqual(0);
+    ed.destroy();
+  });
+});