feat(rebar): rebar strength per 규격 prefix as inputs on ③ (D SD300, H SD400, S SD500)

fyH and fyS become a grade per prefix (D · H · S), chosen from SD300 · SD400 · SD500. The defaults are the P1 values, so
the P1 results do not change. Each bar takes its grade from its 규격 (fyOf). The 철근표 has a 강도 column. Older files
that stored fyH / fyS are read into the grade map. Only fck is still an assumed value (I02). Approved 2026-10-23.

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

index.html

@@ -178,8 +178,9 @@
           <section class="common" aria-labelledby="common-title">
             <p id="common-title" class="lbl">공통 설정</p>
             <label class="mat">fck (MPa) <input id="mat-fck" type="number" step="any" /></label>
-            <label class="mat">fy H · SD400 (MPa) <input id="mat-fyh" type="number" step="any" /></label>
-            <label class="mat">fy S · SD500 (MPa) <input id="mat-fys" type="number" step="any" /></label>
+            <label class="mat">D 규격 강도 <select id="mat-grade-d"><option value="300" selected>SD300</option><option value="400">SD400</option><option value="500">SD500</option></select></label>
+            <label class="mat">H 규격 강도 <select id="mat-grade-h"><option value="300">SD300</option><option value="400" selected>SD400</option><option value="500">SD500</option></select></label>
+            <label class="mat">S 규격 강도 <select id="mat-grade-s"><option value="300">SD300</option><option value="400">SD400</option><option value="500" selected>SD500</option></select></label>
             <p id="mat-note" class="muted small"></p>
           </section>
         </aside>