enable std::error::Error impl for NxError behind std feature

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Enable std::error::Error implementation for NxError when the std feature is active.

Why

• nx-sdk already exposes a std feature
• error interop improves ergonomics in host-side tooling/tests
• implementation appears prepared but currently commented

Scope

• update crates/nx-sdk/src/error.rs
• add conditional impl std::error::Error for NxError under the std feature
• add/adjust tests or compile checks as needed

Done when

• nx-sdk builds cleanly with and without std feature
• NxError integrates with standard error trait under std
• no behavior changes in no-std mode

Notes

Please keep feature-gating explicit and minimal.

Thanks again for considering contributing to Numax.