Function time_mux_bounded 

pub fn time_mux_bounded(a: &Grid, b: &Grid, width: u32) -> BoundedExpr

The combined output of a TIME-MULTIPLEXED array: one physical grid whose every tile is muxed by a ctx selector between fabric a’s config (ctx=1) and fabric b’s config (ctx=0). Because the neighbour channels are themselves ctx-muxed, when ctx=1 the whole array carries a’s dataflow and when ctx=0 it carries b’s — proven in the P3 test. a and b must share dims.