dutchauction/v0 package

Package dutchauction ports the classic Solidity Dutch-auction pattern to gno.land: a seller lists an item at a high starting price that decays linearly, block by block, down to a floor price. The first buyer to call Buy pays whatever the price is at that moment — real ugnot, escrowed by the chain with the transaction and forwarded to the seller on settlement. Overpayment is refunded automatically. State-mutating functions are crossing functions (`cur realm`) per the gno 0.9 interrealm convention.