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-grade.test.ts

@@ -0,0 +1,36 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { DEFAULT_MATERIAL, fyOf, isAssumedMaterial, materialFrom } from '../src/core/rebar/common';
+import { rebarSummary } from '../src/core/rebar/summary';
+import { loadP1 } from './helpers';
+
+describe('③ 철근 강도: 규격 접두어별 입력 (승인 2026-10-23)', () => {
+  it('defaults: D = SD300, H = SD400, S = SD500, the same as the P1 values', () => {
+    expect(DEFAULT_MATERIAL.grade).toEqual({ D: 300, H: 400, S: 500 });
+    expect(fyOf(DEFAULT_MATERIAL, 'H19')).toBe(400);
+    expect(fyOf(DEFAULT_MATERIAL, 'S32')).toBe(500);
+    expect(fyOf(DEFAULT_MATERIAL, 'D22')).toBe(300);
+  });
+
+  it('only fck is assumed now; the grades are confirmed inputs', () => {
+    expect(isAssumedMaterial(DEFAULT_MATERIAL)).toBe(true);
+    expect(isAssumedMaterial({ ...DEFAULT_MATERIAL, fck: 35 })).toBe(false);
+    expect(isAssumedMaterial({ ...DEFAULT_MATERIAL, grade: { D: 300, H: 400, S: 400 } })).toBe(true);
+  });
+
+  it('an older file that stored fyH / fyS is read into the grade map', () => {
+    expect(materialFrom({ fck: 35, fyH: 400, fyS: 400 })).toEqual({ fck: 35, grade: { D: 300, H: 400, S: 400 } });
+  });
+
+  it('a value outside SD300 · SD400 · SD500 falls back to the default for its prefix', () => {
+    expect(materialFrom({ fck: 27, grade: { S: 600 } }).grade.S).toBe(500);
+    expect(materialFrom(undefined)).toEqual(DEFAULT_MATERIAL);
+  });
+
+  it('the P1 values do not change with the defaults (S = SD500 gives the same C7 length)', () => {
+    const model = buildPier(loadP1());
+    const base = rebarSummary(model).rows.find((r) => r.name === 'C7')!.length;
+    const same = rebarSummary(model, materialFrom({ fck: 27, grade: { D: 300, H: 400, S: 500 } })).rows.find((r) => r.name === 'C7')!.length;
+    expect(same).toBe(base);
+  });
+});