release: v0.1.12 — accessibility, Zen mode, scroll sync v2, shortcutHelp plugin

feat(a11y): toolbar keyboard navigation (Arrow keys, Home, End)
feat(a11y): aria-live region for screen reader announcements on mode change
feat(a11y): enhance focus-visible style (outline-offset: 2px, hide on mouse focus)

feat(core): Zen mode (editor.toggleZen() / exec('zen'))
- Toggle with Ctrl+Shift+Z shortcut concept
- Auto-hide toolbar and statusbar, mouse to top edge reveals toolbar
- Centered editor pane with max-width constraint

feat(core): heading-aware scroll sync v2
- Sync preview scroll to nearest heading before cursor position
- Falls back to proportional sync when no headings found

feat(core): Word wrap toggle (editor.toggleWordWrap() / setWordWrap(bool))

feat(plugins): shortcutHelp preset plugin
- Press '?' to open keyboard shortcuts overlay panel
- Lists built-in + user-registered shortcuts
- Click overlay or press Escape to close

style: Zen mode CSS, sr-only class, focus-visible refinement

test: 3 new tests for Zen mode, WordWrap, shortcutHelp (535 total)

chore: bump version 0.1.11 → 0.1.12 across all files

src/themes.js

@@ -1,7 +1,7 @@
 /**
  * MetonaEditor Themes — 主题系统（增强版）
  * @module themes
- * @version 0.1.11
+ * @version 0.1.12
  * @description 全局+实例级主题管理、CSS 变量隔离、外部主题跟随、主题继承
  *
  * v0.1.5 增强：