test: cover user-mode apply atomicity and non-blocking KILL

db: TestSetSessionUserModesIsAtomic installs a trigger that rejects the
is_oper UPDATE after the is_wallops UPDATE has run, proving the '+w-o'
partial-failure the old independent-UPDATE loop exhibited is gone.

ircserver: TestDisconnectDoesNotBlockOnUnresponsiveVictim drives
Disconnect against a net.Pipe victim that never reads and requires the
call to return promptly; verified to fail against the synchronous
implementation. Its counterpart asserts the notification is still
delivered and the socket still closed.

internal/ircserver/export_test.go

@@ -55,3 +55,21 @@ func (s *Server) Stop() {
 func (s *Server) Listener() net.Listener {
 	return s.listener
 }
+
+// NewTestConn wraps an already-established net.Conn in a
+// Conn so tests can drive connection-level behaviour such
+// as Disconnect without standing up a whole server.
+func NewTestConn(
+	log *slog.Logger,
+	cfg *config.Config,
+	tcpConn net.Conn,
+	nick string,
+) *Conn {
+	conn := newConn(
+		context.Background(), tcpConn, log,
+		nil, nil, cfg, nil,
+	)
+	conn.nick = nick
+
+	return conn
+}