Function encode_quant_seg 

pub fn encode_quant_seg(quantizer: &dyn Quantizer) -> Vec<u8> 

Encode a quantizer into the QUANT_SEG binary payload.

Layout:

[quant_type: u8] [tier: u8] [dim: u16 LE] [padding: 60 bytes to 64B]
[type-specific data ...]