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

site/index.html

@@ -421,8 +421,8 @@
       <div class="stat-label">运行时依赖</div>
     </div>
     <div class="stat">
-      <div class="stat-num">390+</div>
-      <div class="stat-label">单元测试</div>
+	    <div class="stat-num">558+</div>
+	      <div class="stat-label">单元测试</div>
     </div>
     <div class="stat">
       <div class="stat-num">80%+</div>