fix(0006e): call RefreshSession after membership changes in all clients (audit 0008 N4)

A secured bus freezes per-subject permissions at connect time, so a peer that
creates or joins a room after connecting cannot pub/sub on it until it reconnects
(RefreshSession). No client called it, so under enforce+ACL the demos failed
closed — pushing the operator to disable the ACL (a security regression at the
operator's discretion).

Wire the membership-change contract into every client:
- cmd/worker: RefreshSession after CreateRoom, before publishing.
- cmd/chat (simple): RefreshSession after CreateRoom+Join, before Subscribe.
- cmd/chat (encrypted demo): A refreshes after CreateRoom; B refreshes after the
  invite+join, both before pub/sub.
- local_files/bridge (gateway): RefreshSession after CreateRoom+Join, before Subscribe.
- mobile: new Session.RefreshSession wrapper + the contract documented for callers.

Contract (documented on the wrappers): after ANY membership change, call
RefreshSession BEFORE pub/sub on the new room (it drops active subs, so it must
precede Subscribe). On an unsecured/dev bus it is a harmless reconnect.

Test:
- TestClientCreateRoomRefreshPublishFlow: end-to-end under enforce+ACL, a peer
  creates a room, refreshes, invites a second peer who joins+refreshes+subscribes,
  and the publish is received — no manual intervention, the ACL stays on.

CGO_ENABLED=0 go build/vet/test green; govulncheck 0 reachable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/worker/main.go

@@ -47,6 +47,13 @@ func main() {
 	if err != nil {
 		log.Fatalf("create room: %v", err)
 	}
+	// Membership-change contract (issue 0006e): the bus freezes per-subject
+	// permissions at connect time, and this room did not exist then. Refresh the
+	// session so the new room's subject becomes publishable under enforce+ACL. On
+	// an unsecured/dev bus this is a harmless reconnect.
+	if err := c.RefreshSession(); err != nil {
+		log.Fatalf("refresh session after create room: %v", err)
+	}
 	log.Printf("room %q -> %s (subject %s, cleartext)", *roomSub, roomID, *roomSub)
 
 	stop := make(chan os.Signal, 1)