Function bounded_fanout_select 

pub fn bounded_fanout_select(
    prefix: &str,
    suffix: &str,
    sel_expr: &str,
    data_wire: &str,
    width: usize,
    slice: usize,
) -> (String, String)

Emit a fan-out-bounded image of the one-bit select sel_expr ? data_wire : 0 over width bits, sliced into slice groups.

Returns (decls, pp_expr):

• decls — the slice (* keep *) select-bit copies, one per line, each named {prefix}{s}{suffix} and driven by sel_expr. The (* keep *) attribute stops synthesis (yosys) from merging the copies back into one high-fan-out net; iverilog ignores it, so simulation is unchanged.
• pp_expr — the concatenation { {G{prefix_{slice-1}_suffix}} & data[hi:lo] , ... , {G{prefix_0_suffix}} & data[hi:lo] } where G = width / slice. Groups are concatenated high→low so group 0 lands at the LSB. No net in this expression drives more than G mux bits — the fan-out is bounded by width / slice, never the full datapath (the met1-antenna root cause).

prefix/suffix frame the select-copy family so copy s is named {prefix}{s}{suffix} (the suffix carries any per-instance tag, e.g. a lane suffix _l3 — pass "" when there is none). data_wire is the wire being gated (e.g. "mc"). Panics unless slice > 0 && width % slice == 0, so the byte-groups tile data_wire exactly with no dropped or misaligned bit.