feat(core,view): ③ 공통 설정 — fck and fy as inputs (spec §6.2)

- Material {fck, fyH, fyS}, defaults 27 / 400 / 500 as assumed values (I02).
  The rebar calculations take the material: the C7/C8 splice, the P2 development
  length, the P3 lap, and the B2 split use it instead of fixed numbers.
- ③ screen: 공통 설정 inputs with a note that the defaults are assumed (I02).
- Project file stores the material as an optional field; older files open with
  the defaults.
- Tests: fck 27 → 35 shortens the laps by √(27/35); the C7 piece follows fck;
  a non-default fy drops I02; the file round trip.

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

tests/rebar-summary.test.ts

@@ -45,7 +45,7 @@ describe('③ 철근표 요약 (P1)', () => {
 
   it('B9: 320 bars; warnings: B2 needs a splice (13646 > 12000)', () => {
     expect(count('B9')).toBe(320);
-    expect(warnings.map((w) => w.code)).toEqual(['RB-F08']);
+    expect(warnings.map((w) => w.code)).toEqual(['I02', 'RB-F08']); // defaults are assumed values (I02)
     // B2 is split into two pieces (추천안): each piece is about half the 13646 bar plus half the lap.
     expect(rows.find((r) => r.name === 'B2')!.length!).toBeCloseTo(7776, 0);
   });