G1: preserve GPU Posterize threshold parity

Use an explicit FMA for the final quotient correction at Posterize
boundaries. Align the kinetic motion-blur QA comparison with export
settings and credit sanyamsingh001.

Co-authored-by: Codex <[email protected]>

crates/gpu/src/shaders/fx_stylize.wgsl

@@ -40,7 +40,7 @@ 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);
-    return q + r / a;
+    return fma(r / a, vec3<f32>(1.0), q);
 }
 
 fn fxs_rem2(i: i32) -> i32 {