Function generate_and_add_piece 

fn generate_and_add_piece(
    is_castle: bool,
    builder: &mut Builder,
    rng: &mut LegacyRandom,
    foot: IVec3,
    dir: Direction,
    depth: i32,
)

Out-of-range branch builds a BridgeEndFiller (consuming RNG for selfSeed) then discards it. We mirror vanilla: call create_piece for RNG sync, don’t add.