feat(core,view): footing rebar B1–B12 (plan C5) and the ③ rebar screen v1

- Footing (확대기초철근 (1).md): grid counts from W and L, layer heights,
  B1–B4 and B5–B8 counts and lengths (B2 13646, B6 11396), B10-1/B10-2,
  B11 (60) and B12 (47) with their lengths. B9 is not computed yet.
- rebarSummary: one 철근표 per pier from the model (bearing, coping, column,
  footing), with alerts: RB-F08 (B2 needs a splice, Q-B6), RB-F09 (B9 not
  computed).
- ③ screen v1: stage nav button, member tabs (받침블럭 · 코핑 · 기둥 · 확대기초),
  rebar table with 번호 · 규격 · 개수 · 길이 · 비고, alert list, and the open
  questions (Q-B5, Q-B6, Q-C9). Drawings and the 강조 연동 are not in v1.
- Tests: footing values from the document; P1 summary totals; warnings.
- Not verified in a browser.

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

index.html

@@ -16,7 +16,7 @@
         <nav class="steps" aria-label="단계">
           <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>
-          <span class="step" aria-disabled="true">③ 철근배근 <em>예정</em></span>
+          <button id="nav-rebar" class="step" type="button">③ 철근배근 <em id="step3-status">예정</em></button>
         </nav>
         <div class="actions">
           <span id="save-status" class="savest">저장 전</span>
@@ -134,6 +134,29 @@
         </aside>
       </main>
 
+      <main class="grid rebar" id="view-rebar" hidden>
+        <aside class="panel piers" aria-labelledby="rebar-member-title">
+          <p id="rebar-member-title" class="lbl">부재</p>
+          <ul id="rebar-tabs" class="members"></ul>
+          <p class="pnote">형상은 ② 형상계획 기준입니다. 형상이 바뀌면 "형상 바뀜"으로 표시합니다.</p>
+        </aside>
+        <section class="panel viewer" aria-labelledby="rebar-title">
+          <div class="viewer-head"><h2 id="rebar-title">철근표</h2></div>
+          <p id="rebar-note" class="num"></p>
+          <div id="rebar-table"></div>
+        </section>
+        <aside class="panel results" aria-labelledby="rebar-issues-title">
+          <h2 id="rebar-issues-title">알림</h2>
+          <ul id="rebar-issues" class="issues"></ul>
+          <h2>검토 필요 (결정 대기)</h2>
+          <ul class="issues">
+            <li>Q-B5 B1·B3·B5·B7 다리 길이 (제안값 사용)</li>
+            <li>Q-B6 B2·B6 이음 위치와 길이</li>
+            <li>Q-C9 P4–B11 간섭: 무시로 결정, 문서 반영 필요</li>
+          </ul>
+        </aside>
+      </main>
+
     </div>
 
     <dialog id="import-dialog" aria-labelledby="dlg-title">