feat: modular HAL, Raylib backend, PBR shading, textures, 60 unit tests

- Replace hardcoded SDL3 windowing with IWindow/IInputState/Key abstractions
- Each render backend owns its window (Raylib GLFW, SDL3 for Vulkan)
- Raylib backend: DrawModelEx, custom GLSL shader with Fresnel, ACES
  tonemapping, gamma correction, hemisphere ambient
- Fix backface culling, mesh memory (NativeMemory.Alloc), texture loading
- Camera controllers use backend-agnostic Key enum (inverted yaw/strafe)
- Demo scene: 8 cubes, 7 spheres, torus knot OBJ with checker texture
- Extract ProceduralMesh + MeshMath from Program.cs to Engine.Graphics
- Vulkan backend deferred (compiles, untested, IWindow-compatible)
- 60 unit tests: ObjLoader, camera controllers, AiCommandProcessor,
  RenderBackendFactory, Timing, ProceduralMesh, MeshMath, Transform
- AGENTS.md for opencode integration

scripts/run.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Run the Cortex Engine.
+# Examples:
+#   ./scripts/run.sh                           # run with defaults
+#   ./scripts/run.sh --mcp-port 5000           # run with MCP HTTP server
+#   ./scripts/run.sh --camera-tour             # capture screenshots and exit
+#   ./scripts/run.sh --mcp-stdio             # run headless stdio MCP server
+
+ENGINE_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+export DISPLAY="${DISPLAY:-:0}"
+export DOTNET_ROOT="${DOTNET_ROOT:-$HOME/.dotnet}"
+export PATH="$DOTNET_ROOT:$PATH"
+
+cd "$ENGINE_DIR"
+exec dotnet run --project "$ENGINE_DIR/src/CortexEngine.App/CortexEngine.App.csproj" -- "$@"