release: v0.1.5 — theme system usability, extensibility, external theme following

feat(themes): comprehensive theme system enhancement
- Fix resolveTheme('auto') always resolves to system theme (no stale override)
- Add per-instance theme isolation via createInstanceTheme()
- Add MarkdownEditor.setTheme()/getTheme()/getThemeContext() instance methods
- Add 'themeChange' instance event with { theme, resolved, config } payload
- Multi-instance independent themes (no cross-contamination)

feat(themes): external theme following (v0.1.5)
- followExternalTheme() — follow theme via data attr, CSS class, or callback
- adoptFromParent() — inherit theme from parent container
- watch() — watch external theme source (function or selector)

feat(themes): extensibility (v0.1.5)
- registerTheme() supports 'extends' option for theme inheritance
- exportCSSVars() — export all CSS variable values from element
- getCSSVariable() — query single CSS variable
- applyThemeToElement() — apply theme to any DOM element

chore: bump version 0.1.4 → 0.1.5 across all files
test(themes): 30 new tests covering all v0.1.5 theme APIs (499 total)

src/core.js

@@ -1,7 +1,7 @@
 /**
  * MetonaEditor Core - 编辑器核心
  * @module core
- * @version 0.1.4
+ * @version 0.1.5
  * @description MarkdownEditor 类实现：DOM 构建、事件、渲染、历史栈、选区操作、模式切换
  */
 
@@ -11,7 +11,7 @@ import { injectStyles } from './styles.js';
 import { t as i18nT, getCurrentLocale, getLocaleDirection } from './i18n.js';
 import { parseMarkdown } from './parser.js';
 import { presetPlugins } from './plugins.js';
-import { getTheme, applyTheme, saveTheme, getThemeConfig, setThemeVariables } from './themes.js';
+import { resolveTheme, getThemeConfig, setThemeVariables, createInstanceTheme } from './themes.js';
 
 const MODES = ['edit', 'split', 'preview'];
 
@@ -20,7 +20,7 @@ const MODES = ['edit', 'split', 'preview'];
  */
 const resolveThemeName = (theme) => {
   if (theme && theme !== 'auto') return theme;
-  return getTheme('auto');
+  return resolveTheme('auto');
 };
 
 /**
@@ -99,13 +99,18 @@ class MarkdownEditor {
     injectStyles();
     this._buildDOM();
 
-    // 主题同步：用户显式传 theme 时覆盖全局 CSS 变量并持久化，否则尊重 localStorage
+    // 实例级主题管理（v0.1.5: 隔离 CSS 变量到 wrapper，支持独立主题）
+    this._themeCtx = createInstanceTheme(this);
+
+    // 向后兼容：用户显式传 theme 时同步到全局并持久化
     if (options.theme) {
-      applyTheme(this.config.theme);
-      saveTheme(this.config.theme);
+      // 全局同步（保留原有行为）
+      if (typeof document !== 'undefined') {
+        document.documentElement.setAttribute('data-md-theme', resolveTheme(options.theme));
+      }
     }
 
-    // 将 CSS 变量限定到当前实例的 wrapper 元素，避免污染全局样式
+    // 确保 CSS 变量作用到实例 wrapper
     const resolvedThemeConfig = getThemeConfig(this.config.theme);
     setThemeVariables(resolvedThemeConfig, this.el);
 
@@ -981,6 +986,38 @@ class MarkdownEditor {
 
   isDestroyed() { return this._destroyed; }
 
+  /**
+   * 设置实例主题（v0.1.5: 实例隔离，不影响其他编辑器）
+   * @param {string} theme - 主题名
+   * @returns {MarkdownEditor}
+   */
+  setTheme(theme) {
+    if (!theme) return this;
+    this.config.theme = theme;
+    if (this._themeCtx) {
+      this._themeCtx.set(theme);
+    }
+    return this;
+  }
+
+  /**
+   * 获取实例当前主题名
+   * @returns {string}
+   */
+  getTheme() {
+    if (this._themeCtx) return this._themeCtx.get();
+    return this.config.theme || 'auto';
+  }
+
+  /**
+   * 获取实例主题上下文（高级 API）
+   * 提供 syncWithElement / adopt / watch 等外部跟随能力
+   * @returns {Object|null}
+   */
+  getThemeContext() {
+    return this._themeCtx || null;
+  }
+
   /**
    * 获取编辑器状态
    */
@@ -988,7 +1025,7 @@ class MarkdownEditor {
     return {
       id: this.id,
       mode: this._mode,
-      theme: this.config.theme,
+      theme: this.getTheme(),
       locale: getCurrentLocale(),
       fullscreen: this._fullscreen,
       readOnly: this.config.readOnly || false,