add marketplace stuff

v0.1/marketplace.wit

@@ -0,0 +1,5 @@
+/// Interface for querying marketplace details and public keys from the host.
+interface marketplace {
+    /// Returns the active Pumpkin Marketplace Ed25519 public key in hex format (if available/cached).
+    get-public-key: func() -> option<string>;
+}

v0.1/plugin.wit

@@ -67,6 +67,8 @@ world plugin {
     import display;
     /// Provides game rules definitions and queries.
     import game-rules;
+    /// Provides marketplace public key access for signature verification.
+    import marketplace;
 
     // --- Plugin-provided exports ---