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
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 are build-tagged. NewMonitor now takes a list of interfaces.

Not verified on a real mac: live netstat parsing, IP_BOUND_IF dialing, Mullvad leak protection.

Model: opus-4-8 (implementation); fable-5-1 (landing commit)

internal/monitor/monitor_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor_test
 
 import (
@@ -8,26 +6,33 @@ import (
 	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
 )
 
+// Interface names and hosts reused across the monitor package tests.
+const (
+	ifaceTest0 = "test0"
+	ifaceTest1 = "test1"
+	ifaceEth0  = "eth0"
+	host8888   = "8.8.8.8"
+)
+
 func TestNewMonitor(t *testing.T) {
 	t.Parallel()
 
-	mon := monitor.NewMonitor("test0", "Test Interface A", "test1",
-		"Test Interface B", "/tmp/test.log")
+	mon := monitor.NewMonitor([]monitor.IfaceSpec{
+		{Name: ifaceTest0, Label: "Test Interface A"},
+		{Name: ifaceTest1, Label: "Test Interface B"},
+	}, "/tmp/test.log")
 
-	if mon.InterfaceA() == nil {
-		t.Fatal("interfaceA is nil")
+	ifaces := mon.Interfaces()
+	if len(ifaces) != 2 {
+		t.Fatalf("interfaces = %d, want 2", len(ifaces))
 	}
 
-	if mon.InterfaceB() == nil {
-		t.Fatal("interfaceB is nil")
+	if ifaces[0].Name != ifaceTest0 {
+		t.Errorf("interfaces[0].Name = %q, want %q", ifaces[0].Name, ifaceTest0)
 	}
 
-	if mon.InterfaceA().Name != "test0" {
-		t.Errorf("interfaceA.Name = %q, want %q", mon.InterfaceA().Name, "test0")
-	}
-
-	if mon.InterfaceB().Name != "test1" {
-		t.Errorf("interfaceB.Name = %q, want %q", mon.InterfaceB().Name, "test1")
+	if ifaces[1].Name != ifaceTest1 {
+		t.Errorf("interfaces[1].Name = %q, want %q", ifaces[1].Name, ifaceTest1)
 	}
 
 	if mon.ICMPTimeout.Milliseconds() != 500 {
@@ -51,12 +56,31 @@ func TestNewMonitor(t *testing.T) {
 	}
 }
 
+func TestNewMonitorSingle(t *testing.T) {
+	t.Parallel()
+
+	mon := monitor.NewMonitor(
+		[]monitor.IfaceSpec{{Name: ifaceEth0, Label: "default route"}}, "")
+
+	ifaces := mon.Interfaces()
+	if len(ifaces) != 1 {
+		t.Fatalf("interfaces = %d, want 1", len(ifaces))
+	}
+
+	if ifaces[0].Name != ifaceEth0 {
+		t.Errorf("interfaces[0].Name = %q, want %q", ifaces[0].Name, ifaceEth0)
+	}
+}
+
 func TestAddHosts(t *testing.T) {
 	t.Parallel()
 
-	mon := monitor.NewMonitor("test0", "Test A", "test1", "Test B", "")
+	mon := monitor.NewMonitor([]monitor.IfaceSpec{
+		{Name: ifaceTest0, Label: "Test A"},
+		{Name: ifaceTest1, Label: "Test B"},
+	}, "")
 
-	mon.AddReachabilityHost("8.8.8.8")
+	mon.AddReachabilityHost(host8888)
 	mon.AddReachabilityHost("google.com")
 
 	if len(mon.ReachabilityHosts()) != 2 {