Add GL error handling, resource fallbacks and GPU timing; speed up lighting

Bug fixes:
- Release static GL objects (UBOs, logo texture) before the context is
  destroyed via new Renderer::shutdown()
- Fix max point/spot light warnings never firing
- Show frame time in ms instead of seconds
- Size the viewport framebuffer in physical pixels on HiDPI displays
- Stop double-flipping OBJ UVs; add flip_uvs option to load_obj for
  models with a top-left UV origin (backpack)

Build:
- Track header dependencies (-MMD -MP) so header edits trigger rebuilds
- Create build/ automatically

Error handling:
- Add gldebug module: KHR_debug output routed to the logger in debug
  builds, object labels, and error/status string helpers
- Validate inputs and check GL errors when creating textures, meshes,
  framebuffers, uniform buffers and shaders; FrameBuffer::resize() and
  Renderer::init() now report failure

Resources:
- Add ResourceCache: shares textures and shaders by path and substitutes
  fallbacks (magenta checkerboard texture, unlit shader, cube mesh) for
  assets that fail to load; specular maps are optional
- load_obj never fails, a broken model becomes a fallback cube
- Renderer draws submeshes with broken materials using the fallback
  material instead of skipping them
- Add Texture::create_from_pixels, Shader::create_from_source and
  Mesh::create_cube
- Cache uniform locations in Shader, looked up with string_view so
  hits don't allocate; missing uniforms warn once

Performance:
- Add CpuTimer and GpuTimer (GL_TIME_ELAPSED, non-blocking readback)
  and a Performance section in the UI with averaged overlay numbers
- Sample material textures once per fragment and apply material colors
  after accumulating lights
- Pass camera position through CameraBlock instead of inverting the
  view matrix per fragment
- Compute the normal matrix on the CPU instead of per vertex
- Skip spot light work for fragments outside the outer cone

README.md

@@ -18,7 +18,6 @@ system-wide**, the rest of the dependencies are already included.
 After that, building comes down to just running the included Makefile.
 
 ```console
-$ mkdir build
 $ make # optionally pass -j$(nproc)
 ```
 
@@ -51,6 +50,7 @@ flowchart TB
         Renderer --> Camera
         Renderer --> Scene
         Renderer -->|Updates| UniformBuffer
+        Renderer -->|Owns| ResourceCache
     end
 
     subgraph World
@@ -89,6 +89,8 @@ flowchart TB
             Material --> Shader
             Material --> Texture
             UniformBuffer -->|Global data| Shader
+            ResourceCache -.->|Caches, falls back| Texture
+            ResourceCache -.->|Caches, falls back| Shader
         end
     end
 
@@ -101,7 +103,7 @@ flowchart TB
     classDef loading fill:#d97706,stroke:#92400e,color:#fff
     classDef platform fill:#64748b,stroke:#334155,color:#fff
 
-    class Renderer,Scene,Camera,Framebuffer application
+    class Renderer,Scene,Camera,Framebuffer,ResourceCache application
     class Entity,Transform,Mesh,SubMesh,Material,LightEnvironment world
     class UniformBuffer,Shader,Texture,Vertices,Indices,DirectionalLight,PointLight,SpotLight data
     class Loader,tinyobjloader,stb_image loading