style(DEN-1592): format the merged Nix bundle writer

[ORESoftware]

Scope

Formatting-only repair for the standalone Nix bundle writer merged in 1b282b94efc994bb9352b572c9652d5a078787f2.

A three-platform downstream gate discovered that current main is not cargo fmt --all --check clean. The observed drift is confined to:

• src/nix_bundle_write.rs
• tests/nix_bundle_write_boundaries.rs
• tests/nix_bundle_write_cli.rs

This branch contains a temporary same-repository materializer that:

1. runs the repository-pinned formatter;
2. fails if any file outside those three changes;
3. compile-checks both Nix bundle test targets;
4. deletes itself; and
5. commits only permanent formatting changes.

The PR must not merge while the temporary workflow remains in the diff. After the bot-authored formatting commit lands, the review delta should be exactly the three Rust files above with no semantic changes.

This repair is intentionally separate from global-package PR #117, so unrelated current-main cleanup is not hidden inside a feature change.

[linear-code]

DEN-1592