GPU: note why Posterize's division correction is an fma (#405 follow-up)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/gpu/src/shaders/fx_stylize.wgsl

@@ -40,6 +40,8 @@ fn fxs_smoothstep(e0: f32, e1: f32, x: f32) -> f32 {
 fn fxs_qdiv(x: vec3<f32>, a: f32) -> vec3<f32> {
     let q = x / a;
     let r = fma(-q, vec3<f32>(a), x);
+    // `q + r / a`, written as an fma so drivers (NVIDIA Vulkan) can't contract it another way
+    // and round differently from the CPU.
     return fma(r / a, vec3<f32>(1.0), q);
 }