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>

tests/validate-and-foundation.test.ts

@@ -181,9 +181,9 @@ describe('꺾인 코핑 IP4 (spec §4.1)', () => {
     expect(solveIp4({ e: 0, n: 0 }, { e: 1, n: 0 }, { e: 10, n: 2 }, 0)).toBeNull();
   });
 
-  it('bent coping is still reported as 미실시 by buildPier', () => {
+  it('a bend on a straight P1 sheet (IP1, IP2, IP3 collinear) is refused: IP4 would land on IP3', () => {
     const p1 = loadP1();
     const ips = p1.ips.map((p) => (p.name === 'IP4' ? { ...p, angleDeg: 5 } : p));
-    expect(() => buildPier({ ...p1, ips })).toThrow('미실시');
+    expect(() => buildPier({ ...p1, ips })).toThrow('꺾임각과 IP 배치');
   });
 });