[client] Index peer tunnel IPs for O(1) PeerStateByIP lookup

Replace the linear scan over all peers with an ipToKey map maintained
by AddPeer/RemovePeer, covering both IPv4 and IPv6 tunnel addresses.

Offline peers are intentionally no longer resolvable by IP: only active
peers can carry traffic, so IdentityForIP and the DNS disconnected-peer
filter now treat them as unknown, same as foreign IPs.

Skip the DNS answer filter for single-record responses; dropping the
only answer was always restored by the empty-answer escape hatch, so
the fast path is behavior-neutral.

client/internal/dns/local/local.go

@@ -482,7 +482,7 @@ func (d *Resolver) logDNSError(logger *log.Entry, hostname string, qtype uint16,
 // completely when every proxy peer is offline (the upstream may still
 // be reachable some other way, or the peerstore may be stale).
 func (d *Resolver) filterDisconnectedPeerAnswers(logger *log.Entry, question dns.Question, records []dns.RR) []dns.RR {
-	if len(records) == 0 {
+	if len(records) < 2 {
 		return records
 	}
 	d.mu.RLock()

client/internal/dns/local/local_test.go

@@ -2738,6 +2738,17 @@ func TestLocalResolver_FilterDisconnectedPeerAnswers(t *testing.T) {
 			connByIP:    nil,
 			wantInOrder: []string{"100.64.0.10", "100.64.0.11"},
 		},
+		{
+			// A single answer is never filtered: dropping it would only
+			// trigger the empty-answer escape hatch, so the fast path
+			// returns it untouched.
+			name:    "single disconnected answer passes through",
+			records: []nbdns.SimpleRecord{disconnectedRec},
+			connByIP: map[string]ipState{
+				"100.64.0.11": {known: true, connected: false},
+			},
+			wantInOrder: []string{"100.64.0.11"},
+		},
 	}
 
 	for _, tc := range tests {