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/broker.rs

@@ -1,6 +1,7 @@
-//! The broker: message routing, mailboxes, subscriptions, the version store,
-//! and the requests to `broker` (spawn, upgrade, rollback, stop, start,
-//! subscribe, unsubscribe, scan, exit). `supervisor.rs` runs each WASM actor.
+//! The broker: message routing, mailboxes, subscriptions, taps, the version
+//! store, and the requests to `broker` (spawn, upgrade, rollback, stop, start,
+//! subscribe, unsubscribe, tap, untap, scan, exit). `supervisor.rs` runs each
+//! WASM actor.
 //!
 //! The broker accepts only `.wasm` files. It does not know any source language.
 //! The world folder is the source of truth: `scan` boots every
@@ -133,17 +134,23 @@ impl Broker {
             Some(own) => format!("{from}/{own}"),
             None => format!("m-{}", self.seq.fetch_add(1, Ordering::Relaxed)),
         };
-        let recipients: Vec<String> = match to {
-            Some(to) => vec![to.to_string()],
-            None => self
-                .metas
-                .lock()
-                .unwrap()
-                .iter()
-                .filter(|(_, m)| m.subscriptions.contains(from))
-                .map(|(id, _)| id.clone())
-                .collect(),
+        let (recipients, tappers) = {
+            let metas = self.metas.lock().unwrap();
+            let recipients: Vec<String> = match to {
+                Some(to) => vec![to.to_string()],
+                None => metas
+                    .iter()
+                    .filter(|(_, m)| m.subscriptions.contains(from))
+                    .map(|(id, _)| id.clone())
+                    .collect(),
+            };
+            let tappers: Vec<String> = metas.iter().filter(|(_, m)| m.tapped).map(|(id, _)| id.clone()).collect();
+            (recipients, tappers)
         };
+        if !tappers.is_empty() {
+            let message = json!({"id": id, "from": from, "to": to, "reply_to": reply_to, "body": body});
+            self.tap(&tappers, message);
+        }
         for to in recipients {
             self.deliver(Envelope {
                 id: id.clone(),
@@ -156,6 +163,25 @@ impl Broker {
         id
     }
 
+    /// Give every tapper a copy of one routed message. Copies skip `send`, so
+    /// they are never tapped themselves; a full mailbox drops the copy.
+    fn tap(&self, tappers: &[String], message: Value) {
+        let ts = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).map_or(0, |d| d.as_millis() as u64);
+        let body = json!({"type": "tapped", "message": message, "ts": ts});
+        let mailboxes = self.mailboxes.lock().unwrap();
+        for tapper in tappers {
+            if let Some(tx) = mailboxes.get(tapper) {
+                let _ = tx.try_send(Envelope {
+                    id: format!("m-{}", self.seq.fetch_add(1, Ordering::Relaxed)),
+                    from: "broker".into(),
+                    to: tapper.clone(),
+                    reply_to: None,
+                    body: body.clone(),
+                });
+            }
+        }
+    }
+
     /// Route one stdout line of a WASM actor.
     pub fn route_line(&self, from: &str, line: &str) {
         let line = line.trim();
@@ -467,6 +493,12 @@ impl Broker {
                 })?;
                 Ok(json!({"type": format!("{ty}d"), "actor": publisher}))
             }
+            "tap" | "untap" => {
+                let on = ty == "tap";
+                self.update_meta(&env.from, |m| m.tapped = on)?;
+                let answer = if on { "tapping" } else { "untapped" };
+                Ok(json!({"type": answer}))
+            }
             "scan" => Ok(json!({"type": "scanned", "changes": self.scan().await})),
             "exit" => {
                 self.log("broker", &format!("exit requested by {}", env.from));
@@ -474,7 +506,7 @@ impl Broker {
                 self.exit.notify_one();
                 Ok(json!({"type": "exiting"}))
             }
-            other => bail!("unknown broker request {other:?}. Known: spawn, upgrade, rollback, stop, start, subscribe, unsubscribe, scan, exit"),
+            other => bail!("unknown broker request {other:?}. Known: spawn, upgrade, rollback, stop, start, subscribe, unsubscribe, tap, untap, scan, exit"),
         }
     }