refactor: make structuredClone pure without globalThis

[antfu]

Make the exported structuredClone function pure by removing its dependency on globalThis.structuredClone. It now always uses the serialize/deserialize implementation path.

• Simplify structuredClone export to a pure function
• Remove globalThis checks and native delegation
• Update documentation to reflect pure implementation
• Remove global polyfill example (no longer delegates to native)

All tests pass.