Function fuel_compact_sign 

pub fn fuel_compact_sign(
    private_key: &[u8; 32],
    digest: &[u8; 32],
) -> Result<[u8; 64], O2Error>

Sign a 32-byte digest and return a 64-byte Fuel compact signature.

The recovery ID is embedded in the MSB of byte 32 (first byte of s). Low-s normalization is applied: if s > order/2, negate s and flip recovery_id.