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

GameCli.Tests/ObservationTests.cs

@@ -0,0 +1,52 @@
+using GameCli;
+using Xunit;
+
+namespace GameCli.Tests;
+
+public class ObservationTests
+{
+    [Fact]
+    public void Build_HasSevenDimensions()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var obs = Observation.Build(ship, target: (0.5f, 0.5f));
+        Assert.Equal(7, obs.Length);
+    }
+
+    [Fact]
+    public void Build_DisplacementIsTargetMinusShip()
+    {
+        var ship = new ShipState(0.3f, 0.4f, 0f, 0f, 0f, 0f);
+        var obs = Observation.Build(ship, target: (0.7f, 0.9f));
+        // dx, dy at indices 0, 1
+        Assert.Equal(0.4f, obs[0], precision: 5);
+        Assert.Equal(0.5f, obs[1], precision: 5);
+    }
+
+    [Fact]
+    public void Build_EncodesVelocity()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0.1f, -0.2f, 0f, 0f);
+        var obs = Observation.Build(ship, target: (0.5f, 0.5f));
+        Assert.Equal(0.1f,  obs[2], precision: 5);
+        Assert.Equal(-0.2f, obs[3], precision: 5);
+    }
+
+    [Fact]
+    public void Build_EncodesAngleAsSinCos()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0f, 0f, MathF.PI / 2f, 0f);
+        var obs = Observation.Build(ship, target: (0.5f, 0.5f));
+        // sin(π/2)=1, cos(π/2)=0
+        Assert.Equal(1f, obs[4], precision: 5);
+        Assert.Equal(0f, obs[5], precision: 5);
+    }
+
+    [Fact]
+    public void Build_LastDimIsAngularVelocity()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0.7f);
+        var obs = Observation.Build(ship, target: (0.5f, 0.5f));
+        Assert.Equal(0.7f, obs[6], precision: 5);
+    }
+}

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,
+        };
+    }
+}