SPIKE: Investigate swapping in mrrc library for pymarc

`mrrc` is a new rust MARC processing library, inspired by `pymarc`, with python bindings that may provide significant performance boosts to our MARC processing flows (~4x, possibly). We need to do some testing and evaluation of suitability.

- [ ] Create a branch of f_m_t
- [ ] Perform necessary refactoring (should be minimal) to swap in `mrrc` for pymarc
- [ ] Perform head-to-head testing

Watch out for:
* `mrrc` was largely a vibe-coding project built with Claude Code, so we need to do some due diligence on code quality
* Consider implementing an abstraction that allows `mrrc` to serve as an optional dependency