Function gen_constants_from_ir 

pub(crate) fn gen_constants_from_ir(
    fields: &[LayoutField<'_>],
    gen_location: bool,
) -> TokenStream

Generate packing constants from layout IR.

This function generates compile-time constants (<FIELD>, <FIELD>_OFFSET, <FIELD>_BYTES) for slot assignments, offsets, and byte sizes based on the layout IR using field-name-based naming. Slot constants (<FIELD>) are generated as U256 types, while offset and bytes constants use usize.