Detect interfaces per platform; add macOS and single-interface support (closes #2)

Linux runs exactly as before when both bridge interfaces exist. When they do
not, the lone default-route interface is monitored, and a single interface now
draws a single UI pane instead of an empty second one.

macOS is newly supported: a running VPN tunnel (utun) is monitored as the
primary pane alongside the physical default-route interface, or the physical
interface alone when no VPN is up.

Detection lives in internal/netdetect: interface/route data types, pure
selection logic keyed on OS name, and route parsers, all unit-tested on Linux
for both platforms. Only the real route query and the per-platform TCP dial
binding (source address on Linux, IP_BOUND_IF on macOS) are build-tagged. Ping
argument construction is a pure, OS-keyed function. NewMonitor now takes a list
of interfaces.

Model: opus-4-8

internal/cli/root.go

@@ -1,18 +1,17 @@
-//go:build linux
-// +build linux
-
 package cli
 
 import (
 	"context"
 	"os"
 	"os/signal"
+	"runtime"
 	"syscall"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
 
 	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
+	"git.eeqj.de/sneak/rtnetmon/internal/netdetect"
 )
 
 // Config holds the application configuration
@@ -73,12 +72,17 @@ func init() {
 	_ = viper.BindPFlag("logfile", rootCmd.Flags().Lookup("logfile"))
 }
 
-func runMonitor(cmd *cobra.Command, args []string) error {
+func runMonitor(cmd *cobra.Command, _ []string) error {
 	monitor.Logf(cfg.LogFile, "Starting rtnetmon")
-	monitor.Logf(cfg.LogFile, "Monitoring interfaces %s and %s", cfg.IfaceA, cfg.IfaceB)
+
+	// Detect which interfaces to monitor for this platform.
+	specs, err := detectInterfaces(cmd)
+	if err != nil {
+		return err
+	}
 
 	// Create the monitor
-	mon := monitor.NewMonitor(cfg.IfaceA, cfg.LabelA, cfg.IfaceB, cfg.LabelB, cfg.LogFile)
+	mon := monitor.NewMonitor(specs, cfg.LogFile)
 
 	// Add reachability hosts
 	for _, host := range cfg.Hosts {
@@ -112,6 +116,43 @@ func runMonitor(cmd *cobra.Command, args []string) error {
 	return mon.Run(ctx)
 }
 
+// detectInterfaces enumerates the host, reads its default routes, and applies
+// the platform selection rules to decide which interfaces to monitor.
+func detectInterfaces(cmd *cobra.Command) ([]monitor.IfaceSpec, error) {
+	ifaces, err := netdetect.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	routes, err := netdetect.DefaultRoutes()
+	if err != nil {
+		return nil, err
+	}
+
+	flags := netdetect.Flags{
+		IfaceA:    cfg.IfaceA,
+		LabelA:    cfg.LabelA,
+		IfaceB:    cfg.IfaceB,
+		LabelB:    cfg.LabelB,
+		LabelASet: cmd.Flags().Changed("labelA"),
+		LabelBSet: cmd.Flags().Changed("labelB"),
+	}
+
+	panes, err := netdetect.Select(runtime.GOOS, ifaces, routes, flags)
+	if err != nil {
+		return nil, err
+	}
+
+	specs := make([]monitor.IfaceSpec, len(panes))
+	for i, p := range panes {
+		specs[i] = monitor.IfaceSpec{Name: p.Name, Label: p.Label}
+	}
+
+	monitor.Logf(cfg.LogFile, "Monitoring %d interface(s)", len(specs))
+
+	return specs, nil
+}
+
 // Execute runs the root command
 func Execute() error {
 	return rootCmd.Execute()