all: implement PingRequest for node connectivity checking

Implement tailcfg.PingRequest support so the control server can verify
whether a connected node is still reachable. This is the foundation for
faster offline detection (currently ~16min due to Go HTTP/2 TCP retransmit
behavior) and future C2N communication.

The server sends a PingRequest via MapResponse with a unique callback
URL. The Tailscale client responds with a HEAD request to that URL,
proving connectivity. Round-trip latency is measured.

Wire PingRequest through the Change → Batcher → MapResponse pipeline,
add a ping tracker on State for correlating requests with responses,
add ResolveNode for looking up nodes by ID/IP/hostname, and expose a
/debug/ping page (elem-go form UI) and /machine/ping-response endpoint.

Updates #2902
Updates #2129

hscontrol/mapper/builder.go

@@ -277,6 +277,12 @@ func (b *MapResponseBuilder) buildTailPeers(peers views.Slice[types.NodeView]) (
 	return tailPeers, nil
 }
 
+// WithPingRequest adds a PingRequest to the response.
+func (b *MapResponseBuilder) WithPingRequest(pr *tailcfg.PingRequest) *MapResponseBuilder {
+	b.resp.PingRequest = pr
+	return b
+}
+
 // WithPeerChangedPatch adds peer change patches.
 func (b *MapResponseBuilder) WithPeerChangedPatch(changes []*tailcfg.PeerChange) *MapResponseBuilder {
 	b.resp.PeersChangedPatch = changes