fix(net): room creator can rejoin a room they left (A8)

The gossip bootstrap list was derived solely from the ticket: a client
dialed the host, but "we are the host" produced an EMPTY list. So when the
room CREATOR rejoined their own room (their ticket names themselves as host)
they dialed nobody and never re-entered the swarm — the remaining peer stayed
stuck until it too left and rejoined. (First 2-human field test, 2026-06-14;
user-confirmed call-breaking, P1.)

Fix: retain the peers seen in the current room across leave (core
`known_peers`, updated by the event task; reset only when the joined ticket
changes) and pass them to `RoomState::join` as extra bootstrap targets. The
new pure `compute_bootstrap` seam unions the ticket host + retained peers,
drops self, and de-dups; address resolution rides the persistent lookup.

- `RoomState::join` gains `extra_bootstrap: Vec<EndpointAddr>` (test_net
  callers pass vec![]).
- +4 unit tests on `compute_bootstrap`, incl. the host-rejoin regression case.

Tests-green (208 lib + 6 + 4 integration), clippy clean. NOT yet 2-machine
field-verified — needs a live host leave→rejoin call.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bin/test_net.rs

@@ -63,7 +63,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         addr: endpoint_a.addr(),
         sharing: None,
     };
-    room_a.join(&ticket_str, state_a).await?;
+    room_a.join(&ticket_str, state_a, vec![]).await?;
     println!("Node A joined topic.");
 
     // Subscribe to events on Node A
@@ -81,7 +81,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         addr: endpoint_b.addr(),
         sharing: None,
     };
-    room_b.join(&ticket_str, state_b).await?;
+    room_b.join(&ticket_str, state_b, vec![]).await?;
     println!("Node B joined topic.");
 
     // Subscribe to events on Node B