[WIP] fix(parquet): preserve explicit mask with sparse pages

[hhhizzz]

Summary

Explicit RowSelectionPolicy::Mask now works when page pruning has loaded only sparse page ranges. Instead of decoding rows whose pages were intentionally skipped, the read plan carries loaded row ranges into a sparse mask cursor and the record batch reader decodes only backed selected segments.

Auto keeps its existing safety behavior by falling back to selectors for unsafe sparse page-loaded masks, so this stays scoped to the mask-preserving correctness path and does not import the broader reference branch's post-filter, cost-model, metrics, benchmark, CI, changelog, or dependency work.

Validation

• cargo fmt --all --check
• git diff --check
• cargo test -p parquet --features arrow --lib selection
• cargo test -p parquet --features arrow,async --test arrow_reader row_filter
• cargo test -p parquet --features arrow,async --test arrow_reader predicate_cache

Post-Deploy Monitoring & Validation

No additional operational monitoring required. This is a parquet reader correctness fix covered by unit and row-filter integration regressions; downstream validation should come from normal CI and parquet reader test coverage.

---