chore: Add nix development environment

[kuznetsss]

High Level Overview of Change

This PR adds a development environment provided by the Nix package manager, along with documentation on how to use it and provides minor updates to the .gitignore file.

Type of Change

• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)

[Copilot]

Pull request overview

This PR adds Nix flakes support to provide reproducible development environments for xrpld. The changes include Nix configuration files, documentation on how to use the Nix development shell, and updates to .gitignore for Nix-related directories.

Changes:

• Adds Nix flake configuration with support for multiple compiler versions (GCC 13-15, Clang 18-21)
• Provides comprehensive documentation for using the Nix development environment
• Updates .gitignore to exclude Nix and direnv artifacts

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
flake.nix	Main Nix flake configuration defining inputs and development shell outputs
flake.lock	Lock file pinning nixpkgs version for reproducibility
nix/utils.nix	Helper utilities for multi-platform support
nix/devshell.nix	Development shell definitions with multiple compiler configurations
docs/build/nix.md	Comprehensive guide for using Nix development environments
docs/build/environment.md	Added reference to Nix as an alternative setup method
.gitignore	Added entries for Nix, direnv, and clangd cache directories
.envrc	direnv configuration for automatic Nix shell activation
.config/cspell.config.yaml	Added Nix-related terms to spell checker dictionary

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 9 changed files in this pull request and generated 5 comments.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 9 changed files in this pull request and generated 5 comments.

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[mathbunnyru]

Is there a pre-commit for .nix files?

[kuznetsss]

Added a simple bash script and a formatter in flakes

[mathbunnyru]

Let's add https://github.com/NixOS/nixfmt as a pre-commit hook, and they officially support it: https://github.com/NixOS/nixfmt#pre-commit
This way it:

• will always work, no matter if nix is installed or not
• won't have inline bash
• will give native pre-commit experience

[kuznetsss]

Their pre-commit hook requires haskell to be installed:

➜ pre-commit run nixfmt --all-files
[INFO] Installing environment for https://github.com/NixOS/nixfmt.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('cabal', 'update')
return code: 1
stdout:
    Executable `cabal` not found
stderr: (none)

[mathbunnyru]

There is another hook, nixfmt-nix, it requires nix to be installed: https://github.com/NixOS/nixfmt/blob/master/.pre-commit-hooks.yaml

But I think it makes sense to require nix to be installed, if you're changing nix files. And it makes sense to have it installed in CI.

Right now, the codestyle is not enabled on the server-side, which means it's not forced.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 10 changed files in this pull request and generated 2 comments.

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