Add reproducible Nix development environment

[jlawrence6809]

Summary

• add a pinned Nix development shell for nxdk and native Linux builds
• default NXDK_DIR to the sibling nxdk checkout while allowing an override
• stage the existing config, images, and font beside the native executable
• document both workflows and ignore the native build directory

This is independent of the file-browser work.

Validation

• nix flake check
• clean native CMake build with all runtime assets staged
• nxdk XBE and XISO build
• independent final review: GO

[jlawrence6809]

Apologies! Agent decided to cut this on it's own. Closing.