gui_toolkit 0.15.0

Added

• A WebGPU render backend (experimental), behind the same interface as the Direct3D 11 one: -define:TOOLKIT_RENDER=wgpu (default d3d11). wgpu-native over Direct3D 12 (or Vulkan with -define:TOOLKIT_WGPU_BACKEND=vulkan); the shaders ported to WGSL; translucent windows through a DirectComposition visual; GPU time per step. The pixel tests pass on both backends (tools\test_backends.ps1). See docs/webgpu.md.
• tools\fetch_wgpu.ps1 (wgpu-native and DXC pinned by SHA-256), tools\render_probe, tools\ppm_diff.ps1, tools\window_shot.ps1.
• Shaders written once, in WGSL: render/rect.wgsl, filter.wgsl, slug.wgsl (the Slug text shaders, ported), mip.wgsl. The HLSL of the Direct3D 11 backend is generated from them by naga (tools/wgsl2hlsl, naga pinned exactly, Cargo.lock pinned by SHA-256 in tools\build_naga.ps1), compiled to DXBC and embedded as before; odin run tools/shaders writes both; tests/shaders checks the SHA-256 of the sources in the header of each HLSL and the blobs; CI runs tools/shaders -- -check. render/rect.hlsl, filter.hlsl, slug.hlsl are gone (generated copies live in render/shaders/). D3D11 pixel tests unchanged, GPU time at a held clock and CPU per frame unchanged, first present +0 to +10 ms.n- render.backend_name(), render.adapter_description(); TOOLKIT_RENDER_TRACE=1` prints the time of the first present.