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)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@metona-team/metona-editor",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "description": "轻量、零依赖、精致美观的 Markdown Editor 库。单文件，开箱即用。",
   "type": "module",
   "main": "dist/metona-editor.js",