task(CAD-4): otrack computeTrackPoint with angle projection, divide/measure path point tools on defpoints in one undo transaction

frontend/src/canvas/SnapEngine.ts

@@ -494,3 +494,44 @@ export class SnapEngine {
 export function fromOffset(base: { x: number; y: number }, dx: number, dy: number): { x: number; y: number } {
   return { x: base.x + dx, y: base.y + dy };
 }
+
+// ────────────────────────────────────────────────────
+// Task CAD-4: OTRACK — Objekt-Fang-Tracking. computeTrackPoint
+// projiziert den Cursor auf Tracking-Linien durch den Basispunkt
+// (Default 0/90/180/270°, optional erweiterte Winkel) und liefert
+// den nächstgelegenen Track-Punkt. Reine Funktion (UI-testbar).
+// ───────────────────────────────────────────────────
+
+export function computeTrackPoint(
+  base: { x: number; y: number },
+  cursor: { x: number; y: number },
+  angles?: number[],
+): { x: number; y: number } | null {
+  const dx = cursor.x - base.x;
+  const dy = cursor.y - base.y;
+  if (dx === 0 && dy === 0) return null;
+  const angs = angles ?? [0, 90, 180, 270];
+  let best: { x: number; y: number } | null = null;
+  let bestDist = Infinity;
+  for (const a of angs) {
+    const rad = (a * Math.PI) / 180;
+    const dxc = Math.cos(rad);
+    const dyc = Math.sin(rad);
+    let pt: { x: number; y: number };
+    if (Math.abs(dxc) >= Math.abs(dyc)) {
+      if (dxc === 0) continue;
+      const t = dx / dxc;
+      pt = { x: cursor.x, y: base.y + t * dyc };
+    } else {
+      if (dyc === 0) continue;
+      const t = dy / dyc;
+      pt = { x: base.x + t * dxc, y: cursor.y };
+    }
+    const dist = Math.hypot(pt.x - cursor.x, pt.y - cursor.y);
+    if (dist < bestDist) {
+      bestDist = dist;
+      best = pt;
+    }
+  }
+  return best;
+}