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

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

src/AcDream.App/Rendering/ChaseCamera.cs

@@ -0,0 +1,66 @@
+using System;
+using System.Numerics;
+
+namespace AcDream.App.Rendering;
+
+/// <summary>
+/// Third-person chase camera that follows behind and above a player
+/// character. Implements <see cref="ICamera"/> so it plugs into the
+/// existing renderer pipeline.
+/// </summary>
+public sealed class ChaseCamera : ICamera
+{
+    public Vector3 Position { get; private set; }
+    public float Aspect { get; set; } = 16f / 9f;
+    public float FovY { get; set; } = MathF.PI / 3f;
+
+    /// <summary>Distance behind the player.</summary>
+    public float Distance { get; set; } = 8f;
+
+    /// <summary>Camera pitch above horizontal (radians). Positive = look down.</summary>
+    public float Pitch { get; set; } = 0.35f;  // ~20 degrees
+
+    /// <summary>Vertical offset from the player's feet to the look-at point (eye height).</summary>
+    public float EyeHeight { get; set; } = 1.5f;
+
+    private const float PitchMin = 0.05f;
+    private const float PitchMax = 1.4f;  // ~80 degrees
+
+    private float _playerYaw;
+    private Vector3 _lookAt;
+
+    public Matrix4x4 View =>
+        Matrix4x4.CreateLookAt(Position, _lookAt, Vector3.UnitZ);
+
+    public Matrix4x4 Projection =>
+        Matrix4x4.CreatePerspectiveFieldOfView(FovY, Aspect, 1f, 5000f);
+
+    /// <summary>
+    /// Update the camera position to follow the player.
+    /// </summary>
+    public void Update(Vector3 playerPosition, float playerYaw)
+    {
+        _playerYaw = playerYaw;
+        _lookAt = playerPosition + new Vector3(0f, 0f, EyeHeight);
+
+        // Camera offset: behind the player (-forward direction) and above.
+        float forwardX = MathF.Cos(playerYaw);
+        float forwardY = MathF.Sin(playerYaw);
+
+        float horizontalDist = Distance * MathF.Cos(Pitch);
+        float verticalDist = Distance * MathF.Sin(Pitch);
+
+        Position = new Vector3(
+            playerPosition.X - forwardX * horizontalDist,
+            playerPosition.Y - forwardY * horizontalDist,
+            playerPosition.Z + EyeHeight + verticalDist);
+    }
+
+    /// <summary>
+    /// Adjust pitch by a delta (from mouse Y movement).
+    /// </summary>
+    public void AdjustPitch(float delta)
+    {
+        Pitch = Math.Clamp(Pitch + delta, PitchMin, PitchMax);
+    }
+}