Function prepare_layer_mips 

pub fn prepare_layer_mips(
    extracted: Res<'_, ExtractedUiLayers>,
    store: ResMut<'_, LayerTextureStore>,
    pipeline: Option<Res<'_, LayerBlitPipeline>>,
    specialized: ResMut<'_, SpecializedRenderPipelines<LayerBlitPipeline>>,
    pipeline_cache: Res<'_, PipelineCache>,
    render_device: Res<'_, RenderDevice>,
    meta: ResMut<'_, LayerMipMeta>,
)

Stage the downsample chain for every wants_mips layer whose sampled texture’s mips are stale. Predictive like the filter staging: mips_valid flips true only when the staged run is certain to execute this frame (source complete + blit pipeline compiled); until then the composite’s bilinear fallback covers.

Runs after prepare_layer_textures (slots/views exist) and prepare_layer_filters (output_valid/output_index decided), before prepare_layer_composites (its bind-group choice reads mips_valid).