Implement Original BND Design: Split `_HITID.BND` Into Separate Bound Files

[KcRobin9]

Currently, bound polygons for all areas are consolidated into a single {map_name}_HITID.BND file.

This implementation differs from the original design, where spatial data was more granularly managed:

1. A primary reference polygon (or polygons) for each specific area / cell (e.g. cell 201) is defined.
2. Detailed polygons for these areas are stored in dedicated bound files (e.g., BOUND201.BND).
3. The {map_name}_HITID.BND file serves primarily as an index, directing the game to load detailed data from the appropriate BOUND file. Should a detailed file be missing, the game defaults to the (minimal) polygon data in _HITID.BND.

See the pictures below for more context.

HITID Polygons:

Separate Polygons: