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

README.md

@@ -2,7 +2,7 @@
 
 > 轻量、零依赖、精致美观的 Markdown Editor 库。单文件，开箱即用，中文优先。
 
-[![npm version](https://img.shields.io/badge/version-0.1.11-blue.svg)](https://www.npmjs.com/package/@metona-team/metona-editor)
+[![npm version](https://img.shields.io/badge/version-0.1.12-blue.svg)](https://www.npmjs.com/package/@metona-team/metona-editor)
 [![license](https://img.shields.io/badge/license-MIT-green.svg)](./LICENSE)
 [![tests](https://img.shields.io/badge/tests-532%20passed-brightgreen.svg)](./tests)
 [![coverage](https://img.shields.io/badge/coverage-97%25-brightgreen.svg)](./tests)