Study checkpointing and resumption

[jc-macdonald]

Problem

Long-running studies can't resume if interrupted. save_results/load_results exist for ResultsTable but Study doesn't persist mid-run state. A multi-phase study that fails in Phase 3 must restart from scratch.

Proposed

• Study.save(path) / Study.load(path) — serialize completed phase results + current phase index
• Auto-checkpoint after each phase completes (opt-in via checkpoint_dir= parameter)
• On Study.run(), skip phases whose results already exist on disk
• Use existing save_results/load_results under the hood