fix(assembly): add DNA/RNA support to to_fasta method

[tpdmskim]

casp data 만들 때 assembly.to_fasta() method 쓰는데
DNA, RNA 지원 안 되어서 추가합니다!

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Note

Low Risk
Small, localized change to sequence export plus additive tests/fixtures; low risk aside from potential edge cases in residue ID mapping for nucleic acids.

Overview
Assembly.to_fasta() now generates FASTA output for all polymer chains (Protein, DNA, RNA) instead of protein-only, using new dna_restype_3to1/rna_restype_3to1 mappings (defaulting unknown bases to N) while preserving protein modres handling.

Adds pytest coverage for protein/DNA/RNA FASTA generation and introduces new mmCIF/CCD test fixtures (notably tests/test_data/mmcif/1bna.cif and additional nucleotide components in components_stdres.cif) to support the new cases.

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[codecov]

Codecov Report

❌ Patch coverage is 10.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.58%. Comparing base (552bec6) to head (33d96b8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gmol/base/data/mmcif/assembly.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   82.88%   82.58%   -0.31%     
==========================================
  Files          14       14              
  Lines        1648     1654       +6     
  Branches      226      229       +3     
==========================================
  Hits         1366     1366              
- Misses        221      227       +6     
  Partials       61       61              

Files with missing lines	Coverage Δ
src/gmol/base/data/mmcif/assembly.py	78.02% <10.00%> (-0.96%)	⬇️

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[tpdmskim]

@jnooree 이것도 test 짜야 하나용?!

[jnooree]

짜면 좋긴 한데 워낙 단순해서.. 걍 합쳐도 될거같슴다