[client] Address review comments on DNS lazy-connection warm-up

- Use netip.Addr instead of string IPs in the local resolver's
  PeerConnectivity and PeerActivator interfaces; extract record
  addresses as netip.Addr (v4-mapped forms unmapped) and convert to
  string only at the peer.Status boundary.
- Scope warm-up to match-only (non-authoritative) zones so the
  synthesized private-service records keep warming while plain
  peer-name lookups in the authoritative peer zone no longer wake
  every idle peer.
- Parse NB_DNS_LAZY_WARMUP_TIMEOUT once in the resolver constructor
  instead of on every request, and log invalid values.
- Stop reusing engine.syncMsgMux on the DNS path: ConnMgr.ActivatePeer
  is now safe for concurrent use (lazy manager pointer guarded by a
  dedicated RWMutex; the manager itself is internally synchronized),
  so network-map processing no longer blocks DNS resolution.
- Add SetPeerActivator to the dns.Server interface (no-op on the mock)
  and drop the *dns.DefaultServer type assertion in the engine.
- Drop WireGuard wording from dns/local comments; the layer is
  transport-agnostic.

Tests: authoritative zones never trigger warm-up; warm-up timeout env
parsing (valid/invalid/non-positive); extractRecordAddr unmaps
v4-mapped record data; dnsPeerActivator skips connected/unknown/
conn-less peers, returns on connect, and releases at the budget when
the peer stays idle; ConnMgr.ActivatePeer races the manager lifecycle
cleanly under -race.

client/internal/engine.go

@@ -657,15 +657,12 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 	// Wire DNS-time lazy-connection warm-up now that the connection manager
 	// exists (it does not at DNS-server construction time). A DNS answer that
 	// points at an idle peer then wakes it before the client's first request.
-	if ds, ok := e.dnsServer.(*dns.DefaultServer); ok {
-		ds.SetPeerActivator(&dnsPeerActivator{
-			connMgr:   e.connMgr,
-			peerStore: e.peerStore,
-			status:    e.statusRecorder,
-			mu:        e.syncMsgMux,
-			ctx:       e.ctx,
-		})
-	}
+	e.dnsServer.SetPeerActivator(&dnsPeerActivator{
+		connMgr:   e.connMgr,
+		peerStore: e.peerStore,
+		status:    e.statusRecorder,
+		ctx:       e.ctx,
+	})
 
 	e.srWatcher = guard.NewSRWatcher(e.signal, e.relayManager, e.mobileDep.IFaceDiscover, iceCfg)
 	e.srWatcher.Start(peer.IsForceRelayed())