Function encode_layer_quantized 

pub fn encode_layer_quantized(
    layer: &ColumnarLayer,
    quantize_m: Option<f64>,
) -> Result<Vec<u8>>

encode_layer with optional fixed-point coordinate quantization.

quantize_m = Some(meters) stores coordinates as i32 grid indices at that ground precision (default-off None is byte-identical to encode_layer). Coordinates are the dominant, near-incompressible tile column, so quantizing them is the single largest size lever — at the cost of GeoArrow Float64 self-description, hence opt-in. The per-layer affine rides in the geometry field metadata under STT_QUANT_META_KEY; the reader reconstructs Float64.

The non-coordinate settings (attribute quantization, vector grouping, point-elevation fold, vertex-time precision) come from the process-wide globals; use encode_layer_with to pass every setting explicitly.