Function flat_styles_rgba8_from_geometry_columns 

pub fn flat_styles_rgba8_from_geometry_columns(
    geom_ids: &[u32],
    geom_colors: &[f32],
    resolved: &ResolvedPrepass,
    decoder: &mut EntityDecoder<'_>,
) -> (Vec<u32>, Vec<u8>)

crate::prepass::flat_styles_rgba8 with the (dominant) geometry-style source supplied as PRE-MERGED columns instead of a map — the sharded finalize path. geom_ids are unique (the host merged shard results first-wins) with geom_colors as rgba f32 quads; resolved carries the support resolution (indexed colours, materials, element colours) and MUST have an empty geometry_style_index. Output is byte-identical to the serial flatten: same precedence (geometry > indexed colour > material > element material), same id-ascending wire order, same rgba8 conversion — without ever building the 4M-entry geometry hashmap.