WIP: TIN terrain system - 95% complete

Replaces heightfield with Cesium Quantized Mesh format (TIN).
Full replacement, not hybrid - TIN is the only terrain format.

Implemented:
- Format: quantizedmeshformat.h, quantizedmeshreader.h
- Triangulator: terraincooker_tin.{h,cpp} using CDT
- Cooker: terraincook now generates TIN by default
- Loader: terraintileloader fully rewritten for TIN
- GPU pool: terraintilepool_tin (VBO/IBO per tile)
- Renderer interface: terrainclipmap.cppm updated
- Shaders: terrainclipmap_tin.{vert,frag}
- Documentation: 5 comprehensive markdown files

Remaining:
- terrainclipmap.cpp render() method (partial, needs completion)
- Build testing and integration
- See TIN_INTEGRATION_GUIDE.md for next steps

Stats: 2700+ lines new code, ~50% vertex reduction expected

shaders/terrainclipmap_tin.frag

@@ -0,0 +1,23 @@
+// TIN Terrain Fragment Shader
+// Minimal material lookup, no blending for now
+
+#version 330 core
+
+// Input from vertex shader
+in vec2 vUV;
+flat in uint vMaterial;
+
+// Uniforms
+uniform sampler2DArray ground;   // Material texture array
+
+// Output
+out vec4 fragColor;
+
+void main() {
+    // Sample material texture
+    // TODO: Add blending between materials for smooth transitions
+    vec3 texCoord = vec3(vUV, float(vMaterial));
+    vec4 color = texture(ground, texCoord);
+    
+    fragColor = color;
+}