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/styles.js

@@ -1,7 +1,7 @@
 /**
  * MetonaEditor Styles - 编辑器样式
  * @module styles
- * @version 0.1.4
+ * @version 0.1.5
  * @description 编辑器 UI 样式注入与管理
  */