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>

src/core/project.ts

@@ -1,3 +1,4 @@
+import type { Material } from './rebar/common';
 import type { PierInput } from './types';
 
 /** Project file schema (spec §8). Bump only with a migration. */
@@ -28,6 +29,8 @@ export interface ProjectFile {
   source: { fileName: string; sheets: string[] };
   piers: ProjectPier[];
   history: { version: string; note: string; at: string }[];
+  /** ③ 공통 설정 (fck · fy). Absent in older files: the defaults are used. */
+  common?: Material;
 }
 
 /** Serialises a project to the pretty-printed JSON stored in <교량명>_교각설계.pzr.json. */