Refactor: per-RG independent reverse scan (modeled after Atlas ReverseParquetSource)

Replace ReversedRowGroupStream's rg_row_counts boundary detection with
per-row-group independent reading. Each RG gets its own
ParquetRecordBatchStreamBuilder with RowFilter applied independently,
then batches are reversed per-RG. This fixes the correctness issue
where RowFilter reduces actual rows below rg_row_counts predictions.

Memory: O(largest RG), same as Atlas's ReverseParquetSource.

Added SLT test for exact reverse + pushdown_filters + predicate.

Author: zhuqi-lucas