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)

README.md

@@ -2,9 +2,9 @@
 
 > 轻量、零依赖、精致美观的 Markdown Editor 库。单文件，开箱即用，中文优先。
 
-[![npm version](https://img.shields.io/badge/version-0.1.4-blue.svg)](https://www.npmjs.com/package/@metona-team/metona-editor)
+[![npm version](https://img.shields.io/badge/version-0.1.5-blue.svg)](https://www.npmjs.com/package/@metona-team/metona-editor)
 [![license](https://img.shields.io/badge/license-MIT-green.svg)](./LICENSE)
-[![tests](https://img.shields.io/badge/tests-469%20passed-brightgreen.svg)](./tests)
+[![tests](https://img.shields.io/badge/tests-499%20passed-brightgreen.svg)](./tests)
 [![coverage](https://img.shields.io/badge/coverage-97%25-brightgreen.svg)](./tests)
 
 ---
@@ -19,7 +19,7 @@
 - **安全** — 内置 XSS 防护（`javascript:`/`vbscript:`/`file:` 协议过滤），HTML 转义，`safeUrl` 净化
 - **内置解析器** — 自研 CommonMark 子集 + GFM 扩展（表格含列对齐、任务列表、上下标、数学公式、脚注、嵌套列表、emoji 短码等），可整体替换
 - **三模式视图** — edit / split / preview 自由切换，分屏拖拽调整比例，滚动同步，模式切换保持滚动位置
-- **主题系统** — light / dark / auto / warm 四套预设，CSS 变量定制，跟随系统主题，@media print 打印样式
+- **主题系统** — light / dark / auto / warm 四套预设，CSS 变量定制，实例级主题隔离，外部主题跟随（syncWithElement / adoptFromParent），主题继承（extends），跟随系统主题，@media print 打印样式
 - **国际化** — zh-CN / en-US 完整翻译，RTL 支持，`Intl` 数字/货币/日期格式化
 - **历史栈** — 撤销/重做，防抖合并（可配置延迟），可配置上限
 - **只读模式** — `readOnly` 配置 / `setReadOnly()` API，适用于预览/展示场景