fix(shaders): enforce y-up screen-space convention + document coordinate conventions

- Add textureSampleYUp() helper to fullscreen_uv_vs.wgsl to correct
  y-flip when sampling WebGPU textures with y-up UV coordinates
- Use textureSampleYUp() in passthrough, gaussian_blur, combined_postprocess
- Fix skybox.wgsl: remove erroneous (1.0 - uv.y) flip (double-flip bug)
- Document world/view/screen conventions in doc/3D.md, camera_common.wgsl,
  and fullscreen_uv_vs.wgsl

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/shaders/gaussian_blur.wgsl b/src/shaders/gaussian_blur.wgsl
index 7f85719..75f8c39 100644
--- a/src/shaders/gaussian_blur.wgsl
+++ b/src/shaders/gaussian_blur.wgsl
@@ -24,7 +24,7 @@ struct GaussianBlurParams {
     for (var i = -kernel_size; i <= kernel_size; i++) {
         let sample_offset = f32(i) * offset;
         let weight = exp(-f32(i * i) / (2.0 * params.radius * params.radius));
-        color += textureSample(input_texture, input_sampler, in.uv + sample_offset) * weight;
+        color += textureSampleYUp(input_texture, input_sampler, in.uv + sample_offset) * weight;
         weight_sum += weight;
     }