release: v0.1.6 — plugin v2, i18n instance isolation, shortcuts, toolbar, toast

feat(plugins): plugin system v2
- depends: declarative plugin dependencies with topological sort (Kahn algorithm)
- async plugins: install() returning Promise auto-await
- editor.unuse(name): uninstall individual plugins
- pluginUtils.validateConfig(schema, config): configuration validation
- plugin priority field controls install order within same dependency level

feat(i18n): instance-level locale isolation, pluralization, dynamic loading
- createInstanceI18n(): per-editor independent locale contexts
- editor.setLocale()/getLocale()/t() instance methods
- Plural rules: t('items', { count: 5 }) auto-selects one/other/few/many
- i18nUtils.loadRemote(url, locale): fetch translation packs from remote
- editor.on('localeChange') event

feat(core): toolbar & shortcut customization, context menu, toast
- editor.registerShortcut(combo, handler): custom keyboard shortcuts
- editor.configureToolbar(tools): dynamic toolbar rebuild
- editor.removeToolbarButton(action): remove single button
- editor.registerContextMenu(items): right-click context menu
- editor.toast(msg, {type, duration, animation}): toast notifications

style: toast notifications, context menu, dropdown CSS
- .me-toast with success/error/warning/info types, fade/slide animations
- .me-context-menu with items, separators, shortcut hints

test: 22 new tests covering plugin deps, unuse, shortcuts, toast, i18n plural (521 total)

chore: bump version 0.1.5 → 0.1.6 across all files, site, types

site/docs.html

@@ -175,7 +175,7 @@
 
 <header class="docs-header">
   <h1><span class="grad">文档</span></h1>
-  <p class="sub">MetonaEditor v0.1.5 完整 API、配置与使用指南</p>
+  <p class="sub">MetonaEditor v0.1.6 完整 API、配置与使用指南</p>
 </header>
 
 <div class="container">
@@ -618,7 +618,7 @@ i18nUtils<span class="c-punc">.</span><span class="c-fn">formatDate</span><span
 
 <footer>
   <div class="container">
-    MetonaEditor v0.1.5 · <a href="https://git.metona.cn/MetonaTeam/MetonaEditor" target="_blank" rel="noopener">源码仓库</a> · MIT License
+    MetonaEditor v0.1.6 · <a href="https://git.metona.cn/MetonaTeam/MetonaEditor" target="_blank" rel="noopener">源码仓库</a> · MIT License
   </div>
 </footer>