Add tuple return support for multiple successive renders

[carldebilly]

Summary

• Handlers can now return C# ValueTuples to produce multiple results rendered separately in sequence
• Each tuple element goes through the full rendering pipeline independently (transformer, colorization, output)
• Works with all return styles: sync, Task<(T1,T2)>, ValueTask<(T1,T2)>, up to 8 elements
• Navigation instructions in tuples are only applied on the last element
• Exit code is determined by the last element

Example

app.Map("status", () => ("Current user: alice", Results.Success("All systems operational")));

This renders two separate outputs: the plain string first, then the success result.