feat(parser): v0.1.15 — parser short+medium term improvements

Short-term fixes:
- Fix ***bold italic*** / ___bold italic___ triple emphasis
- Fix link text inline formatting (recursive renderInline on [**bold**](url))
- Fix backtick inline code nesting (CommonMark-compliant extractInlineCodes)
- Fix tab indentation stripping in indented code blocks

Medium-term improvements:
- parseTokens() / renderTokens() separation API for token-level transform
- registerBlockHandler() table-driven block handler registry
- Backslash escape support (\* \_ \)

Also:
- Add 48 new test cases (583 total, 7 suites, all passing)
- Parser coverage: 95.43% stmts / 91.38% branches / 100% funcs / 98.25% lines
- Update README.md, site/, package.json to v0.1.15

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@metona-team/metona-editor",
-  "version": "0.1.14",
+  "version": "0.1.15",
   "description": "轻量、零依赖、精致美观的 Markdown Editor 库。单文件，开箱即用。",
   "type": "module",
   "main": "dist/metona-editor.js",