fix(shape): 꺾임점 근처 계단 경계를 꺾임점에 맞춘다

계단 경계가 꺾임점에서 블럭 반폭(0.6 m) 이내이면 경계를 꺾임점으로 옮겨, 계단면이 미터면과 같아져 두 구간이
쐐기 틈 없이 만난다 (산외교 P11 후열 ⑥|⑦: 15.07 → 15.14). 양쪽 받침이 꺾임점에서 블럭 반폭 이상 떨어질 때만 옮긴다.

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

src/core/geometry.ts

@@ -334,9 +334,14 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
   if (unequalRows) { frontPlaced.sort((a,b)=>a.g-b.g); backPlaced.sort((a,b)=>a.g-b.g); }
 
   // Step boundaries sit midway between consecutive front bearings in coping coordinates (spec §4.3).
+  // A step boundary less than half a block from the kink moves onto it (2026-11-10), so the step face is the mitre
+  // plane and the two stretches meet without a wedge; only when both bearings stay half a block away from the kink.
+  const halfBlock = DIM.blockSize / 2;
+  const toKink = (bound: number, gLeft: number, gRight: number) =>
+    kinkG !== null && Math.abs(bound - kinkG) < halfBlock && kinkG - gLeft >= halfBlock && gRight - kinkG >= halfBlock ? kinkG : bound;
   const bounds = [0];
   for (let k = 0; k < frontPlaced.length - 1; k++) {
-    bounds.push((frontPlaced[k].g + frontPlaced[k + 1].g) / 2);
+    bounds.push(toKink((frontPlaced[k].g + frontPlaced[k + 1].g) / 2, frontPlaced[k].g, frontPlaced[k + 1].g));
   }
   bounds.push(L);
   // 교좌면 EL per bearing (2026-11-10): when given, it is the coping top of that bearing's step; otherwise z − h1 (z − h2 behind).
@@ -349,7 +354,7 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
   // front bound snaps to it, so slightly staggered rows (P1, ≈ 0.09 m) keep one boundary and no sliver step.
   const backBound = (k: number) => {
     const own = (backPlaced[k-1].g + backPlaced[k].g) / 2;
-    return !unequalRows && Math.abs(own - bounds[k]) < DIM.blockSize / 2 ? bounds[k] : own;
+    return !unequalRows && Math.abs(own - bounds[k]) < halfBlock ? bounds[k] : toKink(own, backPlaced[k-1].g, backPlaced[k].g);
   };
   const backSegments: Segment[] = backPlaced.map((b,k) => ({
     s0: k === 0 ? 0 : backBound(k),