feat(app): add FlyCamera with WASD + mouse look

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.App/Rendering/FlyCamera.cs

@@ -0,0 +1,71 @@
+// src/AcDream.App/Rendering/FlyCamera.cs
+using System.Numerics;
+
+namespace AcDream.App.Rendering;
+
+public sealed class FlyCamera : ICamera
+{
+    public Vector3 Position { get; set; } = new(96, 96, 150);
+    public float Yaw { get; set; } = MathF.PI / 2f;  // facing +Y
+    public float Pitch { get; set; } = -0.3f;         // looking slightly down
+    public float FovY { get; set; } = MathF.PI / 3f;
+    public float Aspect { get; set; } = 16f / 9f;
+
+    public float MoveSpeed { get; set; } = 100f;         // world units per second
+    public float MouseSensitivity { get; set; } = 0.003f;
+
+    private const float PitchLimit = 1.5533f;  // ~89 degrees
+
+    public Matrix4x4 View
+    {
+        get
+        {
+            var forward = Forward();
+            return Matrix4x4.CreateLookAt(Position, Position + forward, Vector3.UnitZ);
+        }
+    }
+
+    public Matrix4x4 Projection
+        => Matrix4x4.CreatePerspectiveFieldOfView(FovY, Aspect, 1f, 5000f);
+
+    /// <summary>
+    /// Integrate position for one frame based on WASD + vertical keys.
+    /// W/S move forward/back in the horizontal plane (ignoring pitch).
+    /// A/D strafe left/right. Up/down translate along world Z.
+    /// </summary>
+    public void Update(double dt, bool w, bool a, bool s, bool d, bool up, bool down)
+    {
+        float step = (float)(MoveSpeed * dt);
+
+        // Forward in the horizontal plane (ignore pitch so W doesn't dive into ground).
+        var flatForward = new Vector3(MathF.Cos(Yaw), MathF.Sin(Yaw), 0f);
+        var right = new Vector3(MathF.Sin(Yaw), -MathF.Cos(Yaw), 0f);
+
+        if (w) Position += flatForward * step;
+        if (s) Position -= flatForward * step;
+        if (a) Position -= right * step;
+        if (d) Position += right * step;
+        if (up) Position += Vector3.UnitZ * step;
+        if (down) Position -= Vector3.UnitZ * step;
+    }
+
+    /// <summary>
+    /// Apply accumulated mouse deltas (pixels since last frame). Positive deltaX
+    /// rotates the view to the right (decreases yaw), positive deltaY rotates
+    /// down (decreases pitch).
+    /// </summary>
+    public void Look(float deltaX, float deltaY)
+    {
+        Yaw -= deltaX * MouseSensitivity;
+        Pitch = Math.Clamp(Pitch - deltaY * MouseSensitivity, -PitchLimit, PitchLimit);
+    }
+
+    private Vector3 Forward()
+    {
+        float cp = MathF.Cos(Pitch);
+        return new Vector3(
+            cp * MathF.Cos(Yaw),
+            cp * MathF.Sin(Yaw),
+            MathF.Sin(Pitch));
+    }
+}