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/index.html

@@ -475,7 +475,7 @@
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         </div>
         <h3>易扩展的插件系统</h3>
-        <p>内置自动保存、导出、查找替换插件；自定义插件只需实现 install / destroy 两个生命周期钩子。</p>
+        <p>插件 v2 支持依赖拓扑排序、异步加载、动态卸载；自定义快捷键与工具栏，右键上下文菜单。</p>
       </div>
       <div class="feature">
         <div class="feature-icon">