feat: v0.2.1 — reference links, context menu, RTL, ja/ko, regex search, hooks, copy API

## Added
- Reference link/image resolution: [text][ref] + ![alt][ref] with [ref]: url definitions
- Right-click context menu: undo/redo/cut/copy/paste/selectAll + custom items
- RTL CSS layout support for Arabic, Hebrew, Persian etc.
- Japanese (ja) and Korean (ko) locales with 60+ keys each
- Divider position localStorage persistence
- Regex search toggle in search/replace panel
- Export HTML with embedded CSS styles
- beforeChange / afterChange lifecycle hooks (instance + global)
- copyAsMarkdown() / copyAsHTML() clipboard APIs
- CHANGELOG.md, CONTRIBUTING.md, CI workflow (.github/workflows/ci.yml)
- 2 new test suites: index.test.ts, styles.test.ts (684 total tests, +74)

## Changed
- autoSave plugin: closure-based state per instance instead of this context
- Plugin install() now receives options as second argument
- RTL locale detection: now uses language prefix (ar-SA → RTL)
- Rollup dev mode: only builds UMD format
- prepublishOnly now includes typecheck + test
- Version bumped to 0.2.1

## Fixed
- [text][ref] now correctly renders as link (was raw text)
- ![alt][ref] no longer produces empty src
- autoSave plugin state isolation across multiple editor instances
- Footnote definitions no longer consumed by refDef handler

tests/plugins.test.ts

@@ -792,8 +792,9 @@ describe('零散分支补全', () => {
     ed.use('autoSave');
     // 让 editor.getValue 抛错，触发 save 的 catch 分支
     jest.spyOn(ed, 'getValue').mockImplementation(() => { throw new Error('get failed'); });
-    const plugin = ed.getPlugins().find((p) => p.name === 'autoSave');
-    expect(() => plugin._save()).not.toThrow();
+    const cleanup = (ed as any).__autoSaveCleanup;
+    expect(cleanup).toBeDefined();
+    expect(() => cleanup.save()).not.toThrow();
     expect(spy).toHaveBeenCalled();
     ed.destroy();
     spy.mockRestore();