feat(view): colour the ③ 3D rebar by diameter or by member, with a legend

Role colours stay the default (spec §6.7). A control on ③ switches to diameter (nominal size, H and S of one size
share a colour, Okabe–Ito palette) or member (받침블럭 · 코핑 · 기둥 · 확대기초). The legend lists only the items
the model has. Highlighting and the state colours are unchanged. Approved 2026-10-23.

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

index.html

@@ -152,6 +152,13 @@
           <p class="pnote">형상은 ② 형상계획 기준입니다. 형상이 바뀌면 "형상 바뀜"으로 표시합니다.</p>
         </aside>
         <div class="center-col">
+          <div class="rebar-color" id="rebar-color" role="group" aria-label="철근 색 기준">
+            <span class="lbl">색 기준</span>
+            <button type="button" data-color="role" aria-pressed="true">역할</button>
+            <button type="button" data-color="diameter" aria-pressed="false">직경</button>
+            <button type="button" data-color="member" aria-pressed="false">부재</button>
+            <ul id="rebar-legend" class="rebar-legend"></ul>
+          </div>
           <div class="viewer-slot" id="slot-rebar"></div>
         <section class="panel viewer" aria-labelledby="rebar-title">
           <div class="viewer-head"><h2 id="rebar-title">철근표</h2></div>