fix mutually canceling orbits

src/lib/ecs/system/ai/OrbitMath.h

@@ -18,24 +18,32 @@ namespace OrbitMath
     constexpr float kOrbitLeadAngle_rad = 0.6f;
 
     // Returns the orbit sense (+1 counter-clockwise, -1 clockwise) that matches
-    // the ship's current movement around `center`, so steering reinforces the
-    // motion the ship already has. When the velocity is nearly radial or near
-    // zero (e.g. a head-on approach or a freshly spawned ship) the sense is
+    // the ship's movement around `center`, so steering reinforces the motion the
+    // ship already has. The sense is taken from the ship's velocity *relative to
+    // the center* (`centerVelocity`): for a moving target this both removes the
+    // target's own motion from the decision and dissolves the degenerate case
+    // where two ships orbiting each other translate in a straight line — there
+    // their shared velocity cancels, leaving ~zero relative velocity. When the
+    // relative velocity is nearly radial or near zero (a head-on approach, a
+    // freshly spawned ship, or that mutual-translation case) the sense is
     // ill-defined; this is an unstable point the ship leaves within a tick or
     // two, so a deterministic fallback of +1 is returned.
     inline float resolveOrbitSign(const QVector2D& shipPos, const QVector2D& center,
-                                  const QVector2D& velocity)
+                                  const QVector2D& velocity,
+                                  const QVector2D& centerVelocity = QVector2D())
     {
         const QVector2D radial = shipPos - center;
+        const QVector2D relativeVelocity = velocity - centerVelocity;
         const float radialLength   = radial.length();
-        const float velocityLength = velocity.length();
+        const float velocityLength = relativeVelocity.length();
         if (radialLength < 1.0e-4f || velocityLength < 1.0e-4f)
         {
             return 1.0f;
         }
 
-        // z-component of radial x velocity, normalised to sin(angle) between them.
-        const float cross = radial.x() * velocity.y() - radial.y() * velocity.x();
+        // z-component of radial x relativeVelocity, normalised to sin(angle).
+        const float cross = radial.x() * relativeVelocity.y()
+                          - radial.y() * relativeVelocity.x();
         const float sinAngle = cross / (radialLength * velocityLength);
 
         constexpr float kRadialEpsilon = 1.0e-3f;