pqkem: strict (fail-closed) mode + wire status Quantum resistance

Strict mode (NB_PQ_MLKEM_STRICT, default off) closes the initial PQ-vulnerable
window (NET-1408): when enabled, conn.presharedKey programs a per-conn random
sentinel PSK until the ML-KEM exchange derives the real one, so no session can form
on a non-PQ key (the real PSK is pushed via SetPresharedKey once it converges).
Default stays opportunistic.

Also surface PQ status: the peer 'Quantum resistance' flag (RosenpassEnabled) is now
true when an ML-KEM PSK has been derived for the peer, not only for Rosenpass.

client/internal/engine.go

@@ -2012,6 +2012,7 @@ func (e *Engine) createPeerConn(pubKey string, allowedIPs []netip.Prefix, agentV
 	}
 	if e.pqkemManager != nil {
 		config.PQ = pqHandshaker{mgr: e.pqkemManager}
+		config.PQStrict = pqkem.Strict()
 	}
 
 	serviceDependencies := peer.ServiceDependencies{