fix(dialer): split netns into platform file to support android cross-compile

internal/net/dialer/dialer_android.go

@@ -0,0 +1,11 @@
+//go:build android
+
+package dialer
+
+import "fmt"
+
+// switchNetns is a no-op stub — network namespace switching is not supported
+// on Android (requires privileged kernel syscalls unavailable to app sandboxes).
+func switchNetns(name string) (restore func(), err error) {
+	return nil, fmt.Errorf("netns not supported on android")
+}