Function select_strategy 

pub fn select_strategy(
    left_estimated_bytes: usize,
    right_estimated_bytes: usize,
    broadcast_threshold_bytes: usize,
) -> JoinStrategy

Determine the join strategy based on estimated data sizes.

Returns Broadcast if the smaller side fits within broadcast_threshold_bytes, otherwise Shuffle. Pass DEFAULT_BROADCAST_THRESHOLD_BYTES when no runtime config is available.