feat: add simple conformer sdf file caching.

[KalenJosifovski]

Summary

Add an opt-in on-disk SDF conformer cache to CASPER.

What changed

• Added cache_sdf and cache_dir to CasperConfig
• Routed featurize(...), featurize_bag(...), and embed_many(...) through a cache-aware embedding path
• Store and reload conformers from deterministic per-molecule SDF cache files
• Preserve MMFF/UFF energies so Boltzmann pooling still works from cached conformers
• Added tests for cache hits, key isolation, corrupt-cache fallback, and energy-preserving reloads
• Updated README and example usage

Impact

Users can avoid regenerating conformers across repeated runs by enabling the cache explicitly.

Validation

• pixi run test: 18 passed, 1 skipped (jazzy not installed), 0 failures
• Verified cache round-trip: cached conformers match fresh embed exactly
• Verified cache survives monkeypatched embed() raising RuntimeError
• Verified Boltzmann pooling works identically from cached and fresh conformers
• Verified corrupt SDF cache silently falls back to fresh embed
• Verified cache key changes when any embedding input (n_confs, seed, prune_rms, optimize) differs
• Verified jagged-bag and NaN-row failure handling in batch APIs

Bug fix

• Fixed jazzy test skip guard: was checking import casper.jazzy_properties (always succeeds), should check import jazzy directly — the dependency is imported lazily at runtime.