feat(parse): report per-sheet recognition and block error sheets

- parseWorkbook returns a report per sheet (table counts, issues, ok)
- sheets with E-level issues cannot be selected (spec section 2.3)
- pier input carries its source sheet name

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

index.html

@@ -42,6 +42,8 @@
       </section>
 
       <section class="results">
+        <h2>엑셀 시트 인식 결과</h2>
+        <ul id="sheets" class="issues"></ul>
         <h2>계산 결과</h2>
         <table id="summary"><tbody></tbody></table>
         <h2>알림</h2>