feat: public parse_fold_chains() for pure-syntactic spec parsing

[DimaMolod]

Summary

Adds parse_fold_chains(spec, protein_delimiter="+") -> List[(name, copies, regions)] — a pure-syntactic parse of a single fold spec into per-chain triples, with no filesystem access and no FeatureIndex lookup.

Why

Downstream tooling (e.g. AlphaPulldownSnakemake length-aware SLURM sizing, KosinskiLab/AlphaPulldownSnakemake#44) needs to know the chain composition of a fold spec at workflow-parse time — before create_features has produced the on-disk feature index that expand_fold_specification / parse_fold require. APS was duplicating the same parse logic; this PR lifts it into the canonical parser package.

What it adds

• New public function parse_fold_chains in alphapulldown_input_parser.parser, re-exported from the package's __init__.
• Internally delegates to the existing _extract_copy_and_regions + _parse_regions, so the name[:copies][:region...] rules stay in one place.
• Chain names are returned verbatim (no path / extension stripping) so callers can normalise as they prefer.
• Examples:

  parse_fold_chains("A+B")             # [('A',1,all), ('B',1,all)]
  parse_fold_chains("A:2:1-100+B")     # [('A',2,1-100), ('B',1,all)]
  parse_fold_chains("/p/protA.json:2") # name kept verbatim, copies=2

Other

• Version bumped 0.4.0 -> 0.5.0.
• README extended with the new helper.
• 26 tests pass (6 new focused on parse_fold_chains).

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