Add dataset sampling strategies

[aviralgarg05]

Summary

Implement dataset sampling strategies for large evaluation sets.

Tasks

• Add DatasetSource.sample(n, strategy) method.
• Support strategies: random, stratified (by metadata field), first-n.
• Ensure reproducibility with seed parameter.
• Document sampling in dataset docs.

Acceptance criteria

• dataset.sample(100, strategy="stratified", field="difficulty") works.
• Same seed produces same sample.
• Original dataset unchanged.