fix: stability improvements — initialize _outlineTimer, extract INLINE_RE constant

- core.js: initialize this._outlineTimer = null in constructor (fixes clearTimeout(undefined))
- parser.js: extract INLINE_RE to module-level constant (avoid recompilation on every scanInline call)

src/core.js

@@ -109,6 +109,7 @@ class MarkdownEditor {
     this._shortcuts = [];          // v0.1.6: 快捷键注册表
     this._contextMenuItems = [];   // v0.1.6: 右键菜单项
     this._customActions = {};      // 自定义工具栏动作
+    this._outlineTimer = null;     // 大纲防抖计时器
 
     // 渲染函数：自定义覆盖内置解析器
     this._renderFn = (typeof this.config.render === 'function') ? this.config.render : parseMarkdown;