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/combined_postprocess.wgsl b/src/shaders/combined_postprocess.wgsl
index c0acfe7..7402284 100644
--- a/src/shaders/combined_postprocess.wgsl
+++ b/src/shaders/combined_postprocess.wgsl
@@ -11,7 +11,7 @@
 
 @fragment fn fs_main(in: VertexOutput) -> @location(0) vec4f {
     // Sample base color
-    var color = textureSample(input_texture, input_sampler, in.uv);
+    var color = textureSampleYUp(input_texture, input_sampler, in.uv);
     
     // Apply effects in sequence (customize as needed)
     // color = apply_solarize(color, 0.4, 0.4, uniforms.time);