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/app.go

@@ -509,6 +509,10 @@ func (h *Headscale) createRouter(grpcMux *grpcRuntime.ServeMux) *chi.Mux {
 		r.Use(h.httpAuthenticationMiddleware)
 		r.HandleFunc("/v1/*", grpcMux.ServeHTTP)
 	})
+	// Ping response endpoint: receives HEAD from clients responding
+	// to a PingRequest. The unguessable ping ID serves as authentication.
+	r.Head("/machine/ping-response", h.PingResponseHandler)
+
 	r.Get("/favicon.ico", FaviconHandler)
 	r.Get("/", BlankHandler)