Macro glue_modules 

macro_rules! glue_modules {
    ($($tk:ident),+ $(,)?) => { ... };
}

Declare a satellite piece’s per-toolkit glue modules — the #[cfg]/#[path] block every piece otherwise hand-writes (docs/extending.md §2). Each named toolkit expands to the house-convention module gate binding lib-<toolkit>.rs next to the invoking lib.rs:

day_pieces::glue_modules!(appkit, gtk, qt, uikit, mdc, xaml);
day_pieces::glue_modules!(uikit, mdc, arkui);   // a piece with partial coverage

Adding a toolkit to Day means one new arm HERE instead of an edit in every piece.