feat(rebar): 철근재료표 by 강종 → 직경 → 번호 with unit weights, 할증 and Excel export

The quantity table sits below the 철근표 for the whole pier. Lines group by grade and nominal diameter, with
subtotals, unit weight 0.00617·d² (kg/m), the total in kg and ton, and a 할증 total. 할증 is an input in 공통 설정,
default 0 as an assumption. Couplers are counted, not weighed. Bars with no length yet (C9, C10, C11, C12, P1, P2) are
listed as missing and kept out of the totals. The table exports to Excel with SheetJS.

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

index.html

@@ -181,6 +181,7 @@
             <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>
+            <label class="mat">재료표 할증 (%) · 기본 0 가정 <input id="mat-waste" type="number" step="0.5" min="0" /></label>
             <p class="lbl">굽힘 내면반경 (× 철근 지름)</p>
             <label class="mat">D16 이하 <input id="bend-small16" type="number" step="0.5" min="0.5" /></label>
             <label class="mat">D19–D25 <input id="bend-main25" type="number" step="0.5" min="0.5" /></label>