Persist NSGA-II parent populations by generation

[Alnusjaponica]

Summary

• Persist selected NSGA-II parent trial IDs in study system attributes.
• Restore valid caches across sampler restarts and recompute stale or incompatible entries.
• Read study attributes once instead of querying storage for every generation.
• Part of Persist NSGA-II parent populations by generation #179, but without additional states.

Tests

• cargo test --workspace
• cargo clippy -p rustuna_sampler --lib --tests -- -D warnings
• cargo fmt --all -- --check

[c-bata]

LGTM!