Fixes #882: add memory guard, bounded A*, and HPA* to prevent OOM in pathfinding

- Add _available_memory_bytes() and _check_memory() to raise MemoryError
  before allocating when the grid would exceed 80% of available RAM
- Add search_radius parameter to a_star_search() for bounded sub-region search
- Auto-compute search_radius when grid exceeds 50% of RAM
- Add HPA* (hierarchical pathfinding) for long paths on large grids:
  coarsen grid, route globally, refine segment by segment
- Apply memory guard and bounded/HPA* dispatch to both numpy and cupy backends
- Add 7 new tests covering bounded A*, memory guard, HPA*, and auto-radius

Author: brendancol