feature: Support Zod v4

[mapsi]

As discussed in #62

I followed the existing patterns to add support for Zod v4

Summary

• Add ModelToZod4 namespace with Zod v4 API changes: z.strictObject(), z.record(z.string(), ...), and z.function({ input, output })
• Rename existing implementation to ModelToZod3
• ModelToZod remains as a backward-compatible alias for ModelToZod3