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>

mobile/unibus.go

@@ -85,6 +85,20 @@ func (s *Session) Join(roomID string) error {
 	return s.c.Join(roomID)
 }
 
+// RefreshSession reconnects the data plane so the bus re-derives this peer's
+// per-subject permissions from its current room membership.
+//
+// Membership-change contract (issue 0006e): a secured bus (--bus-auth enforce)
+// freezes a connection's permissions at connect time. After ANY membership change
+// — a room you just created, were invited to, or joined — call RefreshSession
+// BEFORE Publish/Subscribe on that room, or the bus denies the new room's subject.
+// It also drops active subscriptions, so re-Subscribe afterwards. On an unsecured
+// bus it is a harmless reconnect. A mobile/gateway caller wires this exactly like
+// cmd/chat and cmd/worker do: CreateRoom -> RefreshSession -> Subscribe/Publish.
+func (s *Session) RefreshSession() error {
+	return s.c.RefreshSession()
+}
+
 // Publish sends a UTF-8 text message to the room.
 func (s *Session) Publish(roomID, text string) error {
 	return s.c.Publish(roomID, []byte(text))