nix: convert to flake-parts

[JakeHillion]

Preparing to add a flake-parts module for Rust builds. The flake-utils library doesn't support the module system needed for cleanly separating the Rust build configuration.

Converted from flake-utils to flake-parts, switching to the treefmt-nix flakeModule import pattern. The devShell and formatter outputs remain functionally equivalent.

Test plan:

• Ran nix flake check successfully
• Verified nix develop provides the same environment

[JakeHillion]

Note to reviewers: I'd recommend reviewing this with whitespace disabled in GitHub's diff. Whitespace isn't important in Nix but there's a lot of indentation change.