wgengine, ipn/ipnlocal: route PeerForIP through LocalBackend's live data

userspaceEngine.PeerForIP read from e.netMap.Peers and
e.lastCfgFull.Peers, both of which go stale when peers arrive via
netmap deltas (which skip Engine.SetNetworkMap and Engine.Reconfig).
Every PeerForIP caller (Engine.Ping, the TSMP disco-key handler,
pendopen diagnostics, tsdial.Dialer.UseNetstackForIP, and
LocalBackend.GetPeerEndpointChanges) would report "no matching peer"
for freshly-added peers.

Fix it the same way SetPeerByIPPacketFunc fixed the outbound packet
hot path: have LocalBackend install a callback that reads the live
nodeBackend. nb.NodeByAddr is built from both SelfNode and Peers
(updateNodeByAddrLocked), so a single lookup covers the common case
with IsSelf set when the matched node ID is SelfNode's. The subnet-
route / exit-node-default-route slow path goes through a new
Engine.PeerKeyForIP that exposes the engine's AllowedIPs BART table
(the same table the outbound packet hot path already consults, with
exit-node selection honored), and resolves the matched key back to a
NodeView via the live nodeBackend.

Updates #12542

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I0d4b0d8997c8e796b7367c46b49b61d4fdc717b0

ipn/ipnlocal/state_test.go

@@ -1983,6 +1983,10 @@ func (e *mockEngine) Ping(ip netip.Addr, pingType tailcfg.PingType, size int, cb
 func (e *mockEngine) InstallCaptureHook(packet.CaptureCallback) {}
 
 func (e *mockEngine) SetPeerByIPPacketFunc(func(netip.Addr) (_ key.NodePublic, ok bool)) {}
+func (e *mockEngine) SetPeerForIPFunc(func(netip.Addr) (_ wgengine.PeerForIP, ok bool))  {}
+func (e *mockEngine) PeerKeyForIP(netip.Addr) (_ key.NodePublic, _ netip.Prefix, ok bool) {
+	return key.NodePublic{}, netip.Prefix{}, false
+}
 func (e *mockEngine) SetPeerSessionStateFunc(func(key.NodePublic, wgengine.PeerWireGuardState)) {
 }
 func (e *mockEngine) SetNetLogNodeSource(netlog.NodeSource)                            {}