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-material.test.ts

@@ -27,7 +27,7 @@ describe('③ 공통 설정: fck · fy 입력 (spec §6.2)', () => {
   });
 
   it('a non-default material drops the I02 warning', () => {
-    const codes = rebarSummary(model, { ...DEFAULT_MATERIAL, fyS: 600 }).warnings.map((w) => w.code);
+    const codes = rebarSummary(model, { ...DEFAULT_MATERIAL, fck: 35 }).warnings.map((w) => w.code);
     expect(codes).not.toContain('I02');
   });
 });