feat(app): render landblock with height-ramp shader + orbit camera

Adds a 2-stage GLSL shader (vertex + fragment), a Shader helper that
compiles/links and exposes SetMatrix4 for uniforms, and an OrbitCamera
with yaw/pitch/distance and a 192-unit-centered target for a single
landblock. TerrainRenderer now takes a Shader and issues an actual
DrawElements call with uView + uProjection uniforms. GameWindow owns
the Shader and Camera, routes mouse drag to camera yaw/pitch, and
scroll wheel to camera distance.

The fragment shader maps world Z to a green-brown-white ramp so
lowlands read green, midlands brown, and peaks white — no textures
yet, but enough to visually confirm the terrain shape.

Shaders are copied to the output dir via a <None Update> item group.

Smoke verified against real dats: process stays alive with no GL
errors, no shader compile/link failures, and no exception trail.

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

src/AcDream.App/Rendering/OrbitCamera.cs

@@ -0,0 +1,28 @@
+using System.Numerics;
+
+namespace AcDream.App.Rendering;
+
+public sealed class OrbitCamera
+{
+    public Vector3 Target { get; set; } = new(96, 96, 0);  // center of a 192x192 landblock
+    public float Distance { get; set; } = 300f;
+    public float Yaw { get; set; } = MathF.PI / 4f;
+    public float Pitch { get; set; } = MathF.PI / 6f;
+    public float FovY { get; set; } = MathF.PI / 3f;
+    public float Aspect { get; set; } = 16f / 9f;
+
+    public Matrix4x4 View
+    {
+        get
+        {
+            var eye = Target + new Vector3(
+                Distance * MathF.Cos(Pitch) * MathF.Cos(Yaw),
+                Distance * MathF.Cos(Pitch) * MathF.Sin(Yaw),
+                Distance * MathF.Sin(Pitch));
+            return Matrix4x4.CreateLookAt(eye, Target, Vector3.UnitZ);
+        }
+    }
+
+    public Matrix4x4 Projection
+        => Matrix4x4.CreatePerspectiveFieldOfView(FovY, Aspect, 1f, 5000f);
+}