feat: implement select_dataset() for dynamic dataset filtering

[Kerwin0224]

Summary

Add select_dataset() to RolloutDataSource for dynamic dataset adjustment during RL training.

Changes

• Sample: add global_index field — stable identifier, invariant across rollouts
• Dataset: assign global_index at load time after length filtering
• RolloutDataSource: add select_dataset(), expose full_samples, update save()/load() for checkpoint compatibility

Upstream adjust_dataset() on RolloutManager will call this method — not included in this PR.