- buildAxis: IP1 → IP4 → IP3 as stretches with their own frames; arc length s from IP1; mitre normal u1 + u2 at the kink. - toAxis / fromAxis: world ↔ (k, s, n) round trip; stretch chosen by the mitre plane. - copingSpan (centred on L_IP/2) and nearKink (columns within the Level half-width). - Straight P1 regression against spec §12 bearing values; bent round trip and continuity. - Not wired into buildPier yet; the model still refuses bent coping (미실시). Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>