Reuse Parquet OffsetIndex objects and direct-plan decisions

### Problem

Selective Parquet reads repeat CPU work within one file reader: Arrow 17 reparses OffsetIndex on each lookup, page planning scans page locations for sparse row selections, and decoding rebuilds a direct-read plan just to determine whether compressed row coordinates apply.

The byte cache in #272 reduces repeated I/O but does not eliminate this parsing and planning work.

### Proposed improvement

- Reuse parsed OffsetIndex objects within each retained row-group reader.
- Seek across page gaps for sparse selections and visit each selected page once.
- Reuse the direct-plan decision during decoding.

Preserve index validation, dictionary handling, per-leaf row coordinates and missing-index fallbacks. Keep parsed objects reader-local without introducing a shared data cache or public option.

### Validation

Check selected values and index lifetimes, including sparse/dense selections and the row-group retention limit. Compare against main with the format benchmark, reporting latency and storage bytes without assuming a general speedup.

Implementation: #314.

首先檢查 issue #314，該 issue 被指定為這項工作的實作。驗證 sparse 和 dense 選取中的選定值和索引生命週期，包括 row-group 保留限制，然後比較 format benchmark 與 main 的延遲和儲存位元組數，不要假設會有普遍的加速效果。

由索引模型根據 Issue 內容生成。