Update folding_stability.zip: villin + shared sampling data (flat layout)

Rebuild folding_stability.zip so it works with the merged mlipaudit changes (folding_stability + sampling villin PR).

Changes vs the current archive:
- Rename villin_capped.* -> villin_capped_solvated.{xyz,pdb} and villin_capped_ref.pdb -> villin_capped_solvated_ref.pdb, to match the STRUCTURE_NAMES entry `villin_capped_solvated` in the merged code (box CRYST1 34.199^3, solvated topology, unsolvated _ref).
- Add the sampling dihedral references backbone_reference_data.json and sidechain_reference_data.json. The sampling benchmark now reads its input data from the folding_stability directory (data_name = "folding_stability"), so this archive is the shared superset for both biomolecule benchmarks.
- Remove orexin_beta_1cq0_nmr.* (replaced by villin) and amyloid_beta_1ba6_nmr.* (not in the benchmark's system list).
- Flat layout (no starting_structures/ or pdb_reference_structures/ subdirs) and stripped of .DS_Store / __MACOSX entries.

Contents: chignolin_1uao_xray, trp_cage_2jof_xray, villin_capped_solvated -> each {.xyz, .pdb, _ref.pdb}; plus backbone_reference_data.json and sidechain_reference_data.json.

Opened as a PR for review; do not merge until validated.

folding_stability.zip

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