fix(shape): 받침 번호가 축 반대 방향이어도 계단을 g 순서로 만든다

전·후열 받침 수가 같으면 받침을 번호 순서로 두어, 번호가 IP1 → IP3 반대로 매겨진 교각(산외교 P9: ① g 12.1,
③ g 1.3)에서 계단 경계가 거꾸로(9.42 → 4.02) 놓이고 코핑 상면이 틀렸다. 받침을 늘 g 순서로 정렬한다.

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

src/core/geometry.ts

@@ -331,7 +331,9 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
   };
   const frontPlaced = front.map((b) => ({ no: b.no, row: '전열' as Row, z: b.z, ...place(b) }));
   const backPlaced = back.map((b) => ({ no: b.no, row: '후열' as Row, z: b.z, ...place(b) }));
-  if (unequalRows) { frontPlaced.sort((a,b)=>a.g-b.g); backPlaced.sort((a,b)=>a.g-b.g); }
+  // Rows run along the coping (2026-11-10): bearing numbers may run against IP1 → IP3 (산외교 P9 ① at g 12.1, ③ at 1.3),
+  // so the steps are always built in g order, not number order.
+  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