feat(12-01): add RF monitor collector, WIRELESS_REGISTRATIONS stream, wire into poll cycle

- RFMonitorStats struct for per-interface RF data (noise floor, channel width, TX power)
- CollectRFMonitor with v6/v7 RouterOS version routing
- WIRELESS_REGISTRATIONS NATS stream with 30-day retention (separate from DEVICE_EVENTS)
- WirelessRegistrationEvent type and PublishWirelessRegistrations method
- Poll cycle collects per-client registrations and RF stats, publishes combined event

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poller/internal/device/rf_monitor_test.go

@@ -0,0 +1,32 @@
+package device
+
+import (
+	"testing"
+)
+
+func TestRFMonitorStatsFields(t *testing.T) {
+	// Compilation test: ensure RFMonitorStats has all required fields
+	// with correct types.
+	stats := RFMonitorStats{
+		Interface:         "wlan1",
+		NoiseFloor:        -105,
+		ChannelWidth:      "20MHz",
+		TxPower:           24,
+		RegisteredClients: 15,
+	}
+	if stats.Interface != "wlan1" {
+		t.Error("Interface field not set correctly")
+	}
+	if stats.NoiseFloor != -105 {
+		t.Error("NoiseFloor field not set correctly")
+	}
+	if stats.ChannelWidth != "20MHz" {
+		t.Error("ChannelWidth field not set correctly")
+	}
+	if stats.TxPower != 24 {
+		t.Error("TxPower field not set correctly")
+	}
+	if stats.RegisteredClients != 15 {
+		t.Error("RegisteredClients field not set correctly")
+	}
+}