feat(app): Phase B.2 — ChaseCamera (third-person follow camera)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Rendering/ChaseCameraTests.cs

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+using System;
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public class ChaseCameraTests
+{
+    [Fact]
+    public void Position_BehindPlayer_WhenYawIsZero()
+    {
+        var camera = new ChaseCamera { Aspect = 16f / 9f };
+        camera.Update(playerPosition: Vector3.Zero, playerYaw: 0f);
+
+        // Yaw=0 means facing +X (cos(0)=1, sin(0)=0).
+        // Camera should be BEHIND the player: negative X direction.
+        Assert.True(camera.Position.X < -1f, $"Camera X={camera.Position.X} should be behind player (negative X)");
+        Assert.True(camera.Position.Z > 0f, $"Camera Z={camera.Position.Z} should be above player");
+    }
+
+    [Fact]
+    public void Position_BehindPlayer_WhenYawIsHalfPi()
+    {
+        var camera = new ChaseCamera { Aspect = 16f / 9f };
+        camera.Update(playerPosition: Vector3.Zero, playerYaw: MathF.PI / 2f);
+
+        // Yaw=PI/2 means facing +Y. Camera should be behind: negative Y.
+        Assert.True(camera.Position.Y < -1f, $"Camera Y={camera.Position.Y} should be behind player (negative Y)");
+    }
+
+    [Fact]
+    public void Position_FollowsPlayerPosition()
+    {
+        var camera = new ChaseCamera { Aspect = 16f / 9f };
+        var playerPos = new Vector3(100f, 200f, 50f);
+        camera.Update(playerPosition: playerPos, playerYaw: 0f);
+
+        // Camera should be near the player, not at the origin.
+        Assert.InRange(camera.Position.X, 85f, 100f);  // behind but close
+        Assert.InRange(camera.Position.Y, 195f, 205f);  // roughly same Y
+    }
+
+    [Fact]
+    public void PitchAdjustment_ChangesHeight()
+    {
+        var camera = new ChaseCamera { Aspect = 16f / 9f };
+        camera.Update(playerPosition: Vector3.Zero, playerYaw: 0f);
+        float z1 = camera.Position.Z;
+
+        camera.AdjustPitch(0.2f);
+        camera.Update(playerPosition: Vector3.Zero, playerYaw: 0f);
+        float z2 = camera.Position.Z;
+
+        Assert.True(z2 > z1, "Increasing pitch should raise the camera");
+    }
+
+    [Fact]
+    public void ImplementsICamera()
+    {
+        ICamera camera = new ChaseCamera { Aspect = 16f / 9f };
+        camera.ToString();  // just proves interface is implemented
+    }
+}