perf(shaders): drop nonuniformEXT on text font slots

fontTextureSlot/fontSamplerSlot are push constants, hence provably
dynamically uniform. Wrapping them in nonuniformEXT forced the
per-invocation divergent-descriptor path and blocked hoisting the
uniform descriptor load out of the per-pixel glyph loop.

Removing the decoration is zero correctness risk — it is purely a
read-index hint. Not applied to ui-images.comp.glsl, whose slots come
from an SSBO load the compiler cannot prove uniform.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shaders/ui-text.comp.glsl

@@ -36,9 +36,12 @@ void main() {
         vec2 t  = (sp - it.rect.xy) / it.rect.zw;
         vec2 uv = mix(it.uv.xy, it.uv.zw, t);
 
+        // Font slots are push constants — provably dynamically uniform, so no
+        // nonuniformEXT: lets the compiler hoist the descriptor load out of the
+        // per-pixel glyph loop and avoid the divergent-descriptor path.
         float sdf = texture(
-            sampler2D(uiTextures[nonuniformEXT(pc.fontTextureSlot)],
-                      uiSamplers[nonuniformEXT(pc.fontSamplerSlot)]),
+            sampler2D(uiTextures[pc.fontTextureSlot],
+                      uiSamplers[pc.fontSamplerSlot]),
             uv
         ).r;