broker: tap/untap = receive a copy of every routed message

Tapped copies {type:tapped, message:{id,from,to,reply_to,body}, ts} are
delivered directly (never tapped themselves), dropped when the tapper's
mailbox is full, and the tap is saved in actor.json like subscriptions.

cli/src/world.rs

@@ -5,7 +5,7 @@
 //! <world>/
 //!   actor/<id>/
 //!     actor.wasm        the current version
-//!     actor.json        version, running, subscriptions (broker)
+//!     actor.json        version, running, subscriptions, tapped (broker)
 //!     source/           source of the current version
 //!     public/           written by the actor, read-only for others
 //!     data/             state, writable only by the actor
@@ -43,6 +43,9 @@ pub struct ActorMeta {
     /// The actors whose published messages this actor receives.
     #[serde(default)]
     pub subscriptions: BTreeSet<String>,
+    /// Whether this actor gets a copy of every routed message (`tap`).
+    #[serde(default)]
+    pub tapped: bool,
 }
 
 fn yes() -> bool {