feat(fmt/mmcif): implement mmCif parser

[jnooree]

Checklist

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same issue?
• Have you linked the issue(s) you are working on?

If the change is related to the source code, tests, or build environments, please also check the following:

• Does ./scripts/run_clang_tools.sh pass without any warnings?
• Have you built the project locally without any warnings and errors?
• Do all tests (if new tests are added, including the new ones) pass?

If you added new feature(s), then also check the following:

• Did you also add corresponding tests?

Linked Issues

Link the tracking issue(s) of this PR, with the auto-close keywords here:

---

[codecov]

Codecov Report

Attention: Patch coverage is 85.58952% with 66 lines in your changes missing coverage. Please review.

Project coverage is 91.15%. Comparing base (41b9985) to head (80742b0).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fmt/mmcif.cpp	81.56%	66 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
- Coverage   91.36%   91.15%   -0.22%     
==========================================
  Files          54       56       +2     
  Lines       12943    13318     +375     
==========================================
+ Hits        11826    12140     +314     
- Misses       1117     1178      +61     

Files with missing lines	Coverage Δ
include/nuri/fmt/cif.h	98.05% <100.00%> (+1.35%)	⬆️
src/fmt/fmt_internal.h	100.00% <100.00%> (ø)
src/fmt/pdb.cpp	92.32% <100.00%> (-0.44%)	⬇️
src/fmt/mmcif.cpp	81.56% <81.56%> (ø)

... and 1 file with indirect coverage changes