Feature request: Nix flake / dev shell for building jcode from source

Feature Request: Nix development environment for the repo

origin/master has no flake.nix, flake.lock, or Nix-based dev environment. NixOS users building jcode from source have to set up a pinned Rust toolchain and native dependencies manually, and there's no equivalent of a shell.nix/flake.nix to reproduce the exact toolchain, native libs (OpenSSL, mold, etc.), and build flags used for local development.

What I'd like

A Nix flake that provides a development environment, not a Nix build of jcode itself:

• Pinned Rust toolchain (matching whatever version the project builds against, ideally nightly if the parallel rustc front-end is used).
• Native build dependencies: OpenSSL, mold linker, pkg-config, and whatever else cargo build needs on Linux/NixOS.
• Separate shells for different build targets, since desktop pulls in far more (font shaping, GPU deps) than the TUI alone:
  • a small default shell for TUI-only builds
  • an optional larger shell for desktop builds
  • an optional "full" shell with clippy, rustfmt, rust-src, and other tooling for contributors
• Cargo itself should keep using its normal ~/.cargo registry cache and the repo's target/ directory. Nix should supply toolchain and system libs only, not wrap cargo build in a sandboxed derivation, so incremental builds stay fast.
• A helper script (or documented nix develop invocation) that self-dev / scripts/dev_cargo.sh-style workflows can call so selfdev build-style commands work correctly from inside the Nix shell.
• Docs (e.g. docs/NIXOS.md) explaining the fast-start flow, what's cached vs not, and how to enter the environment via nix develop or a wrapper script.

Why

Building jcode from source on NixOS currently means hand-rolling the toolchain and native deps outside of Nix's usual reproducibility guarantees. A flake-based dev shell would make selfdev builds work out of the box on NixOS with a pinned, reproducible environment, matching what non-NixOS contributors get from rustup/system packages.

Notes

Happy to share a prototype flake and dev-shell wrapper scripts I put together locally if useful as a starting point, rather than a full patch, since jcode doesn't take outside contributions.

Start by checking how cargo build and selfdev build-style workflows are currently invoked, along with the native dependencies and build targets used for TUI and desktop development. Add the requested flake.nix, flake.lock, development shells, helper invocation, and docs/NIXOS.md; done means NixOS contributors can enter a pinned environment with nix develop and build incrementally using the normal Cargo cache and target directory.