Function initiate_tile_readbacks 

pub fn initiate_tile_readbacks(
    extractor: ResMut<'_, TerrainExtractor>,
    pending: ResMut<'_, PendingReadbacks>,
    gpu: Res<'_, TerrainRendererGPU>,
    textures: Res<'_, RenderAssets<GpuTexture>>,
    render_instance: Res<'_, RenderInstance>,
)

Render phase: copy each queued tile to a staging buffer and start async mapping. Must run after the paint compute pass so the copy captures post-compute data. This ordering is enforced in [PaintProcessorPlugin].