feat(core,view): bent coping stage 3 — footings and piles in stretch frames

- Footings, blindings and piles on a bent axis are placed in their stretch's
  frame; 일체기초 is grouped per stretch, so a combined footing never crosses
  the kink (separate footings, each aligned with its stretch).
- Footings, blindings and piles are trimmed at the mitre plane like the coping.
- The stage-2 refusal for bent foundations is removed; the Level-half-width
  refusal for columns at the kink remains.
- Tests: T형 footing crossing the kink stays in stretch 0; 2주형 with columns
  on both sides gives one footing per stretch, with mitre clipping in the scene.

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

src/core/validate.ts

@@ -6,7 +6,6 @@ const EXPECTED_COLUMNS: Record<PierType, number> = { 'T형': 1, '2주형': 2, '3
 /** Rules from spec §10 that are not implemented in this stage. Shown to the user, never silently skipped. */
 export const NOT_IMPLEMENTED_RULES = [
   'E04 fck 미입력 (배근 단계, 3단계)',
-  '꺾인 코핑의 확대기초·말뚝 (3단계; 현장타설말뚝만 지원)',
   'W06 속성 복사 건너뜀 (2단계)',
   'W08 솔플레이트 T1−T2 검사 (2단계)',
 ] as const;