feat(core): lateral coping step (Q1·Q2) and IP4 bend-point solve (spec §4.1)

- Front and back coping tops that differ by 5 mm or more in any segment are
  drawn as two coping parts split at the midline between the bearing rows.
  Tolerance 5 mm and midline position are the draft assumptions (Q1·Q2).
- Scene extrudes each coping part across its own width.
- solveIp4 computes IP4 as the intersection of the IP1→IP2 line with the
  line through IP3 turned by the bend angle (positive = counter-clockwise,
  sign convention to be confirmed). Bent coping geometry remains 미실시.

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

src/core/validate.ts

@@ -6,6 +6,7 @@ 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 코핑 형상 (IP4 꺾임각 ≠ 0; IP4 계산식만 구현)',
   'W06 속성 복사 건너뜀 (2단계)',
   'W08 솔플레이트 T1−T2 검사 (2단계)',
 ] as const;