Modernize type hints to PEP 604/built-in generics across the codebase (grid/neighbors/subset/plot/dataset modules), replacing Optional/Union/List/Dict/Tuple/Set with | unions and built-in generics

Old style (pre-PEP 604):

  - Union[str, int]
  - Optional[bool] (alias for Union[bool, None])

New style (PEP 604, Python 3.10+):

  - str | int
  - bool | None