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>

tests/rebar-member-fck.test.ts

@@ -23,7 +23,7 @@ describe('② 부재별 fck (spec §6.2 · ②)', () => {
     const common = { ...DEFAULT_MATERIAL, fck: 30 };
     expect(materialFor(common, { coping: 35 }, 'coping').fck).toBe(35);
     expect(materialFor(common, { coping: 35 }, 'column').fck).toBe(30);
-    expect(materialFor(common, { coping: 35 }, 'coping').fyS).toBe(common.fyS);
+    expect(materialFor(common, { coping: 35 }, 'coping').grade).toEqual(common.grade);
   });
 
   it('a coping fck changes only the coping rows (C15 stair legs use the H19 development length)', () => {