feat(gamecli): implement Observation.Build with unit tests

GameCli/Observation.cs

@@ -0,0 +1,25 @@
+namespace GameCli;
+
+/// <summary>
+/// Builds the 7-dim observation vector fed to PPO:
+/// [dx, dy, vx, vy, sin(angle), cos(angle), angular_velocity]
+/// Angle is encoded as sin/cos to avoid the discontinuity at ±π.
+/// </summary>
+public static class Observation
+{
+    public const int Dim = 7;
+
+    public static float[] Build(ShipState s, (float X, float Y) target)
+    {
+        return new float[Dim]
+        {
+            target.X - s.X,
+            target.Y - s.Y,
+            s.VX,
+            s.VY,
+            MathF.Sin(s.Angle),
+            MathF.Cos(s.Angle),
+            s.AngularVelocity,
+        };
+    }
+}