Add gateway diagnostics: iperf3, CLI toolkit, periodic WAN speed test

iperf3 serves throughput tests from every VLAN and the mesh; an hourly
librespeed run feeds speedtest_* metrics through node_exporter's textfile
collector, and vmalert flags download rates below half the link's own
7-day median so ISP degradation surfaces without per-site thresholds.

modules/router/iperf.nix

@@ -0,0 +1,28 @@
+# iperf3 server on every gateway, for throughput testing from any LAN segment
+# (e.g. validating AP/switch links: `iperf3 -c 10.<siteId>.<vlan>.1`) and from
+# admin machines over the mesh. Never reachable from the WAN (default-deny).
+{
+  config,
+  lib,
+  ...
+}:
+let
+  cfg = config.cnx.router;
+  mesh = import ../mesh-hosts.nix { inherit config lib; };
+  vlanIfs = lib.mapAttrsToList (name: _: "vlan-${name}") cfg.vlans;
+in
+{
+  config = lib.mkIf cfg.enable {
+    services.iperf3.enable = true;
+
+    networking.firewall.interfaces = lib.genAttrs vlanIfs (_: {
+      allowedTCPPorts = [ 5201 ];
+      allowedUDPPorts = [ 5201 ];
+    });
+
+    networking.firewall.extraInputRules = ''
+      ip6 saddr ${mesh.subnet} tcp dport 5201 accept comment "iperf3 over the mesh"
+      ip6 saddr ${mesh.subnet} udp dport 5201 accept comment "iperf3 over the mesh"
+    '';
+  };
+}