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>

src/core/rebar/coping.ts

@@ -2,7 +2,7 @@
  * 코핑 철근 (plan C3). Source: RAG/코핑철근.md, 2026-10-22 최종 정리 (spec §6.4 is older and is not used here).
  * Positions are along the coping length from its start, in mm.
  */
-import { DEFAULT_MATERIAL, developmentLength, hookLength, Material, NOMINAL_DIAMETER } from './common';
+import { DEFAULT_MATERIAL, developmentLength, fyOf, hookLength, Material, NOMINAL_DIAMETER } from './common';
 export const COPING_REBAR = {
   /** 피복 (최외측 철근 중심까지). */
   cover: 100,
@@ -144,7 +144,7 @@ export const C14_INSET = COPING_REBAR.cover + (NOMINAL_DIAMETER.H25 + NOMINAL_DI
 
 /** Development length l_d of H19 (882 for fy 400, fck 27), rounded to mm as the document does (§7.2). */
 export function stepDevLength(m: Material = DEFAULT_MATERIAL): number {
-  return Math.round(developmentLength(NOMINAL_DIAMETER.H19, m.fyH, m.fck));
+  return Math.round(developmentLength(NOMINAL_DIAMETER.H19, fyOf(m, 'H19'), m.fck));
 }
 
 export interface C14Bar {