Improve partition box key hashing

[yuqian19870725-creator]

Summary

• replace the linear implicit size_t conversion with a dedicated KeyHash
• hash each coordinate independently to reduce unordered-map bucket collisions
• add regression coverage for coordinates that previously collided and for 16³ distinct box lookups

Validation

• MSVC v143 standalone compile/runtime check passed using the updated Partition.h
• 16³ distribution check improved from 106 distinct old hashes to 4,096 distinct new hashes
• the full CMake configure could not complete because the project triplet requires the unavailable Visual Studio v141 toolset