[RFC] Add Nix Flake for Development and Distribution

Summary

Add a flake.nix to enable building and developing jcode via Nix, following modern best practices.

Motivation

• Provide a reproducible build system for Nix users
• Enable nix develop for contributors with a ready-made dev environment
• Support NixOS distribution via flake inputs
• Alternative to existing installation methods (Homebrew, install scripts)

Existing Work

A community fork (faldor20/jcode#1) already has a working flake implementation that could be referenced:

• Uses crane for Rust builds
• Includes devShell with rust-analyzer, rustfmt, clippy
• CI checks (clippy + cargo fmt)
• Sets required build env vars for git metadata in build.rs

Proposed Approach

1. Add flake.nix to root of repository
2. Use crane for deterministic builds with dependency caching
3. Include:
   • packages.default - build jcode binary
   • devShells.default - development environment
   • checks - clippy + fmt validation
4. Document in README under "Installation" section

System Dependencies

• Linux: openssl, libxcb, libX11
• macOS: AppKit, Security, SystemConfiguration (system libs)

Questions for Maintainer

• Should we also pursue adding to official nixpkgs? (requires Cargo.lock in repo)
• Preferred approach for versioning/updates?

References

• faldor20/jcode#1 (draft flake)
• nixpkgs contributing guidelines

Start by reviewing the proposed flake in faldor20/jcode#1 and the repository's existing installation methods, then inspect the README Installation section and Cargo.lock status. The work is done when the root flake.nix provides the default package, devShell, and clippy/format checks, and the README documents Nix development and installation.