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/geometry.ts

@@ -84,6 +84,13 @@ export interface PileModel {
   diameter: number;
 }
 
+/** One coping slab across [nFrom, nTo]; profile is its (s, EL) outline, closed. */
+export interface CopingPart {
+  nFrom: number;
+  nTo: number;
+  profile: [number, number][];
+}
+
 export interface PierModel {
   name: string;
   lengthIp: number;
@@ -106,8 +113,8 @@ export interface PierModel {
   h1: number;
   h2: number;
   segments: Segment[];
-  /** Coping elevation outline as (s, EL) points, closed. */
-  profile: [number, number][];
+  /** Coping split across the width (n range) by the lateral step line; one part when there is no step. */
+  copingParts: CopingPart[];
   levelBottom: number;
   endBottom: number;
   bearings: BearingLocal[];
@@ -129,6 +136,22 @@ export interface PierModel {
   };
 }
 
+/**
+ * Bend point IP4 (spec §4.1-2): the point where the IP1→IP2 line meets the line through IP3 whose
+ * direction is IP1→IP2 turned by the bend angle. Positive angle = counter-clockwise [확인 필요: 부호 규약].
+ * Returns null for a zero angle, where IP4 is not defined.
+ */
+export function solveIp4(ip1: Vec2, ip2: Vec2, ip3: Vec2, angleDeg: number): Vec2 | null {
+  if (angleDeg === 0) return null;
+  const cross = (a: Vec2, b: Vec2) => a.e * b.n - a.n * b.e;
+  const len = Math.hypot(ip2.e - ip1.e, ip2.n - ip1.n);
+  const d1: Vec2 = { e: (ip2.e - ip1.e) / len, n: (ip2.n - ip1.n) / len };
+  const rad = (angleDeg * Math.PI) / 180;
+  const d2: Vec2 = { e: d1.e * Math.cos(rad) - d1.n * Math.sin(rad), n: d1.e * Math.sin(rad) + d1.n * Math.cos(rad) };
+  const t = cross({ e: ip3.e - ip1.e, n: ip3.n - ip1.n }, d2) / cross(d1, d2);
+  return { e: ip1.e + t * d1.e, n: ip1.n + t * d1.n };
+}
+
 function toLocal(p: Vec2, origin: Vec2, u: Vec2, nrm: Vec2): { s: number; n: number } {
   const dx = p.e - origin.e;
   const dy = p.n - origin.n;
@@ -150,7 +173,7 @@ export function buildPier(input: PierInput): PierModel {
   const ip3 = ip('IP3');
   const ip4 = ip('IP4');
   if (ip4.angleDeg !== 0) {
-    throw new Error('미실시: 꺾인 코핑(IP4 꺾임각 ≠ 0)은 아직 지원하지 않습니다.');
+    throw new Error('미실시: 꺾인 코핑(IP4 꺾임각 ≠ 0)의 형상은 아직 지원하지 않습니다. IP4 계산식(solveIp4)은 구현되어 있습니다.');
   }
 
   const dx = ip3.x - ip1.x;
@@ -166,6 +189,7 @@ export function buildPier(input: PierInput): PierModel {
 
   const L = input.copingLengthMm / 1000;
   const B = input.copingWidthMm / 1000;
+  const planHalfB = B / 2;
   const ip1v: Vec2 = { e: ip1.x, n: ip1.y };
   const bp1: Vec2 = { e: ip1v.e + u.e * (lengthIp / 2 - L / 2), n: ip1v.n + u.n * (lengthIp / 2 - L / 2) };
   const bp2: Vec2 = { e: bp1.e + u.e * L, n: bp1.n + u.n * L };
@@ -190,13 +214,9 @@ export function buildPier(input: PierInput): PierModel {
   bounds.push(L);
   const segments: Segment[] = frontLocal.map((b, k) => ({ s0: bounds[k], s1: bounds[k + 1], top: b.z - h1 }));
 
-  // Lateral step: front and back coping tops must match segment by segment (not supported yet).
-  frontLocal.forEach((f, k) => {
-    const backTop = backLocal[k].z - h2;
-    if (Math.abs(segments[k].top - backTop) > DIM.lateralTolerance) {
-      throw new Error('미실시: 횡방향 단차(전열·후열 상면 차 ≥ 5 mm)는 아직 지원하지 않습니다.');
-    }
-  });
+  // Lateral step (spec §4.3, Q1·Q2 추천안): front and back tops differ by 5 mm or more in some segment.
+  const backTops = backLocal.map((b) => b.z - h2);
+  const lateralStep = backTops.some((top, k) => Math.abs(segments[k].top - top) >= DIM.lateralTolerance);
 
   // Columns (spec §4.5). Multi-column positions are placed; haunch and separate-footing checks are not.
   const colS = columnPositions(input.type, L);
@@ -220,11 +240,22 @@ export function buildPier(input: PierInput): PierModel {
     underside.push([zoneStart - DIM.haunch, lowerEl], [next[1] + DIM.haunch, lowerEl], [next[1], levelBottom]);
   });
 
-  const profile: [number, number][] = [[0, endBottom]];
-  for (const seg of segments) {
-    profile.push([seg.s0, seg.top], [seg.s1, seg.top]);
-  }
-  profile.push(...underside);
+  // Elevation outline for one row of tops, closed by the shared underside.
+  const outline = (tops: number[]): [number, number][] => {
+    const pts: [number, number][] = [[0, endBottom]];
+    segments.forEach((seg, k) => pts.push([seg.s0, tops[k]], [seg.s1, tops[k]]));
+    pts.push(...underside);
+    return pts;
+  };
+  // The lateral step line lies midway between the front and back bearing rows.
+  const meanN = (rows: BearingLocal[]) => rows.reduce((sum, b) => sum + b.n, 0) / rows.length;
+  const frontTops = segments.map((seg) => seg.top);
+  const copingParts: CopingPart[] = lateralStep
+    ? [
+        { nFrom: -planHalfB, nTo: (meanN(frontLocal) + meanN(backLocal)) / 2, profile: outline(frontTops) },
+        { nFrom: (meanN(frontLocal) + meanN(backLocal)) / 2, nTo: planHalfB, profile: outline(backTops) },
+      ]
+    : [{ nFrom: -planHalfB, nTo: planHalfB, profile: outline(frontTops) }];
 
   const footingTop = input.foundationType === '현장타설말뚝' ? input.footingBottomEl : input.footingBottomEl + DIM.footingH;
   const columns: ColumnModel[] = colS.map((s) => ({
@@ -294,7 +325,7 @@ export function buildPier(input: PierInput): PierModel {
     h1,
     h2,
     segments,
-    profile,
+    copingParts,
     levelBottom,
     endBottom,
     bearings: [...frontLocal, ...backLocal].map((b) => ({ no: b.no, row: b.row, s: b.s, n: b.n, z: b.z })),

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;

src/view/scene.ts

@@ -65,11 +65,13 @@ export function buildPierScene(m: PierModel): PierScene {
     parts[part].add(new THREE.Mesh(geometry, material));
   };
 
-  // Coping: the elevation outline extruded across the width (no boolean operations).
-  const shape = new THREE.Shape(m.profile.map(([s, z]) => new THREE.Vector2(s, z)));
-  const coping = new THREE.ExtrudeGeometry(shape, { depth: m.copingB, bevelEnabled: false });
-  coping.translate(0, 0, -m.copingB / 2);
-  add('coping', coping);
+  // Coping: each part's elevation outline extruded across its width (no boolean operations).
+  for (const part of m.copingParts) {
+    const shape = new THREE.Shape(part.profile.map(([s, z]) => new THREE.Vector2(s, z)));
+    const coping = new THREE.ExtrudeGeometry(shape, { depth: part.nTo - part.nFrom, bevelEnabled: false });
+    coping.translate(0, 0, part.nFrom);
+    add('coping', coping);
+  }
 
   for (const c of m.columns) {
     const g = new THREE.CylinderGeometry(c.diameter / 2, c.diameter / 2, c.top - c.bottom, 48);