feat(core): 일체기초 footings for 2·3주형 with per-footing pile grid

- Columns closer than one footing length (10400) share a 일체기초 whose
  length is the outer-column distance + 10400, centred between them (spec §4.6).
- Pile grid is computed from each footing's length (rows across 8150,
  pile count along the length from (length − 1400) / 2250 + 1).
- Blinding concrete follows each combined footing.

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

src/core/validate.ts

@@ -8,7 +8,6 @@ export const NOT_IMPLEMENTED_RULES = [
   'E04 fck 미입력 (배근 단계, 3단계)',
   'W06 속성 복사 건너뜀 (2단계)',
   'W08 솔플레이트 T1−T2 검사 (2단계)',
-  '일체기초 판정 (2·3주형)',
 ] as const;
 
 /** Input-level checks (spec §10 E01, E02). Returns false when the pier must not be built. */