Function quantize_position 

pub fn quantize_position(x: FixedPoint) -> i32

High-resolution quantization for node-identity position signatures: 2^20 steps per unit (coordinates live inside the unit disk, so the result fits an i32 with room to spare).

Node identity must NOT use quantize_1000: at 1/1000 resolution two depth-2 cousins in different branches can quantize to identical signatures at a few hundred nodes (silent data crossover). 2^-20 resolution pushes the same birthday bound past millions of nodes, and rainbow fan-out banding guarantees the true positions are distinct.