Function modulate_gmsk 

pub fn modulate_gmsk(
    bits: &[u8],
    n_bits: usize,
    bt: f32,
    sps: usize,
    out_i: &mut [f32],
    out_q: &mut [f32],
)

Modulates packed bits using GMSK.

• bt: Gaussian bandwidth-time product (e.g. 0.25, 0.3, 0.5)
• sps: samples per symbol (must be ≤ 32)

Writes output_len(n_bits, sps) samples to out_i and out_q.