Fix cross-compilation: Add stub for NewUserNamespaceLinux on non-Linux platforms

The macOS build was failing because NewUserNamespaceLinux is only defined
for Linux builds but referenced in the enhanced jail factory. Adding a stub
file that returns an appropriate error for non-Linux platforms.

Author: blink-so[bot]

namespace/user_namespace_stub.go

@@ -0,0 +1,10 @@
+//go:build !linux
+
+package namespace
+
+import "fmt"
+
+// NewUserNamespaceLinux is not available on non-Linux platforms
+func NewUserNamespaceLinux(config Config) (Commander, error) {
+	return nil, fmt.Errorf("user namespace jail not available on this platform")
+}