fix(mesh/outbox): expire messages with zero TTL immediately

is_expired used age > ttl_secs, so a message with ttl_secs=0 whose age
rounded to 0 seconds was considered live forever. Switch to >= so the
zero-TTL boundary expires on the first check, matching the intuitive
meaning of TTL and the behavior the tests assert.

core/archipelago/src/mesh/outbox.rs

@@ -52,7 +52,10 @@ impl PendingMessage {
             return true; // Can't parse = treat as expired
         };
         let age = chrono::Utc::now().signed_duration_since(created);
-        age.num_seconds() as u64 > self.ttl_secs
+        // Use `>=` so a ttl_secs=0 message is expired immediately (used by
+        // tests and by callers that want a fire-and-forget behavior when
+        // the relay can't deliver on first try).
+        age.num_seconds() as u64 >= self.ttl_secs
     }
 
     /// Check if this message can be relayed further.