make workspace build and test reproducibly

- add Gitea sparse registry config for rmath
- commit Cargo.lock
- add default-members so host tests skip GTK/wasm frontends
- fix checkers example to use DraughtsBoard::get
- add README with prerequisites and per-frontend commands
- add Gitea CI workflow (core, wasm, wasm4, MSRV)
- fix repository metadata and stop tracking generated wasm dist

rdraught-gtk/examples/ai_debugger.rs

@@ -2,6 +2,7 @@ use glib::ExitCode;
 use rdraught::{DraughtsBoard, DraughtsGame, Move, Piece, Player, Position};
 use rdraught_gtk::run;
 use std::collections::HashMap;
+use std::mem::size_of;
 
 fn main() -> ExitCode {
     println!("move: {}", size_of::<Move>());