gnoslinger/v0 package

Package gnoslinger is the on-chain lobby and referee of Gunslinger showdowns (the 1982 Avalon Hill gunfight game) of two to four players. Players create and join games and play their turn plans hidden: each commits the hash of its plan, then reveals it once every plan is committed (DESIGN_HIDDEN_PLANS.md); the realm resolves each turn with the pure engine package and keeps the result for the web client to animate.