feat: widen chain IDs from u8 to String for multi-char CIF support

[adobles96]

Summary

• Widens chain ID storage from u8 to String throughout the pipeline to support multi-character chain IDs (e.g. "10" in auth_asym_id from large cryo-EM structures like PDB 9A1O)
• Updates AtomVector, Structure, CompactStructure chain fields and ResidueMatch type accordingly
• CIF parser: replaces get_one_char with get_text_string to preserve full chain names
• PDB/FCZ parsers: convert at boundary, Atom struct unchanged for FFI compat
• Query format: underscore separator (e.g. AA_250) for unambiguous multi-char chains, with backward compat for single-char legacy format (A250)

Test plan

• Tested with large cryo-EM structures (PDB 9A1O) that have multi-char chain IDs
• Verified backward compatibility with single-char chain ID queries

🤖 Generated with Claude Code

[adobles96]

Probably need to propagate the new motif format through documentation if approved. I can do that after/if you green light. I introduced the separator b/c there may be multichar chain IDs with numbers (eg A1_250), I think eg gemmi does this sometimes.

[khb7840]

Ah I was thinking of the same query format with underscore separator with backward compatibility.
Thank you for PR. I'll review this version.

[adobles96]

@khb7840 just bumping this. Are you still interested in merging this?

[khb7840]

Yes. Sorry for the delaying.

[khb7840]

I found the default output format has been changed with your fork and the underscore between the chain ID and the residue indices is not compatible with the query grammar. To keep backward compatibility, I think the output formatting can be kept as the old version by default & new formatting activated with flags or numeric ID discovered. I'm also considering both format to be enabled in querying grammer. Could you make a pull request to widen-chain-ids instead of master?

// BEFORE
AF-O15391-F1-model_v4.pdb	4	84.0118	0.2132	A313,A318,A331,A335	F207,F212,F225,F229
// AFTER
AF-O15391-F1-model_v4.pdb	4	84.0118	0.2132	A_313,A_318,A_331,A_335	F_207,F_212,F_225,F_229

And thank you for the contribution.