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/project.test.ts

@@ -64,3 +64,12 @@ describe('project file round trip (spec §8, plan B3)', () => {
     expect(reopened.piers[0].draft).toBeUndefined();
   });
 });
+
+describe('③ 공통 설정 in the project file', () => {
+  it('fck and fy are kept through save and open; older files get the defaults through the app', () => {
+    const p = sampleProject();
+    const withCommon = { ...p, common: { fck: 35, fyH: 400, fyS: 500 } };
+    expect(parseProject(serializeProject(withCommon)).common).toEqual({ fck: 35, fyH: 400, fyS: 500 });
+    expect(parseProject(serializeProject(p)).common).toBeUndefined();
+  });
+});