feat(view): ④ 재료표 as its own result stage, with pier selection, Excel per pier and print

The 철근재료표 moves from ③ to a new stage ④ in the header. It shows the checked piers (or the current one), one table
per pier with 강종 → 직경 → 번호 and subtotals, and a total across piers. The Excel export has one sheet per pier and a
합계 sheet. Print uses A4 landscape without the header and buttons. ③ keeps a summary line and a link to ④.
Spec §5, §6.9 and §7 record the stage (approved 2026-10-23).

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

index.html

@@ -17,6 +17,7 @@
           <button id="nav-shape" class="step current" type="button" aria-current="step">① 형상 생성 <em id="step1-status">미생성</em></button>
           <button id="nav-plan" class="step" type="button">② 형상계획 <em id="step2-status">예정</em></button>
           <button id="nav-rebar" class="step" type="button">③ 철근배근 <em id="step3-status">예정</em></button>
+          <button id="nav-quantity" class="step" type="button">④ 재료표 <em id="step4-status">예정</em></button>
         </nav>
         <div class="actions">
           <span id="save-status" class="savest">저장 전</span>
@@ -144,6 +145,21 @@
         </aside>
       </main>
 
+      <main class="grid quantity" id="view-quantity" hidden>
+        <aside class="panel piers" aria-labelledby="quantity-pier-title">
+          <p id="quantity-pier-title" class="lbl">교각 목록</p>
+          <ul id="quantity-pier-items" class="pier-items"></ul>
+          <p class="pnote" id="quantity-note"></p>
+        </aside>
+        <section class="panel viewer" aria-labelledby="quantity-title">
+          <div class="viewer-head">
+            <h2 id="quantity-title">철근재료표</h2>
+            <button id="quantity-export" class="btn-o" type="button">엑셀로 내보내기</button>
+            <button id="quantity-print" class="btn-o" type="button">인쇄</button>
+          </div>
+          <div id="quantity-body"></div>
+        </section>
+      </main>
       <main class="grid rebar" id="view-rebar" hidden>
         <aside class="panel piers" aria-labelledby="rebar-pier-title">
           <p id="rebar-pier-title" class="lbl">교각 목록</p>