Add multi-RG tests and fix RG iteration order

Fix: per-RG iteration was popping from Vec back, yielding forward
order instead of reversed. Changed to VecDeque::pop_front to
correctly iterate RGs in reversed order (highest RG first).

Added SLT tests:
- 12.10: multi-RG exact reverse (max_row_group_size=3, 10 rows = 4 RGs)
  - Full DESC scan: correct order across all RG boundaries
  - LIMIT spanning RG boundaries
- 12.11: multi-RG + pushdown_filters + predicate + LIMIT

Author: zhuqi-lucas