Port webgpu_app to render graph

[MatzeOGH]

Ports the webgpu app onto the render graph added in #250. The frame is now
built by declaring resources and passes each frame instead of the previous hand-ordered
rendering.

Merge #250 first.

Changes

• webgpu/engine/Window builds the graph each frame and drives execute().
• The renderers and overlays declare their passes and resources through the graph:
  atmosphere, clouds, track, and the height-lines, screen-space-snow, texture, and
  tile-debug overlays, plus OverlayRenderer and the shared Overlay interface.
• RenderGraphPanel is a debug UI panel for inspecting the compiled graph (passes,
  resources, ordering, culling), plumbed in through ImGuiManager and RenderingContext.
• App.cpp and apps/webgpu_app/CMakeLists.txt register the panel.
• Small additions to webgpu/base/gpu_utils used by the integration.

Files

• App: App.cpp, ImGuiManager.{h,cpp}, RenderingContext.{h,cpp},
  ui/RenderGraphPanel.{h,cpp}, CMakeLists.txt
• Engine: Window.{h,cpp}, the atmosphere/cloud/track renderers, the overlays and
  OverlayRenderer, webgpu/base/gpu_utils.{h,cpp}

[adam-ce]

Failing pipeline are unrelated unit tests. Review will be for Gerald :)