Grid spatial search index

Summary

This MR improves nearest-cell search and nearest-neighbor regridding for large and irregular grids, and cleans up the grid helper/module structure introduced along the way.

It includes:

• an exact and batched nearest-active-cell search path
• a reusable KD-tree spatial index for Cartesian and spherical coordinates
• a dedicated nearest_regridder_t for arbitrary nearest-neighbor remapping
  • target-mask derivation based on source-cell containment
• a refactor of grid-independent helpers into mo_grid_helper
• a new shared mo_grid_constants module
• API cleanup/documentation for the helper module
• a NetCDF nearest-neighbor example and new regression coverage

Main changes

• accelerate exact grid_t%closest_cell_id for regular grids and add batched exact lookup support
• add reusable spatial_index_t in mo_spatial_index for exact nearest-neighbor search in Cartesian and spherical space
• add grid_t%build_spatial_index for grid-backed index construction
• add nearest_regridder_t in mo_grid_scaler for arbitrary source/target nearest-neighbor remapping
• add optional derive_target_mask to the nearest regridder, now based on source-cell containment rather than nearest masked source centers
• add grid_t%in_cell(i,j,x,y,aux) for regular and aux-grid containment checks
• add 04_nc_nearest.f90 and accompanying NetCDF test files for nearest-neighbor regridding
• move grid-independent helpers, data_t, and NetCDF helper routines into mo_grid_helper
• restore legacy mo_grid compatibility exports for data_t, mask_from_var, and data_from_var
• introduce mo_grid_constants as the shared home for grid selector constants
• shorten mo_grid_helper public helper names by dropping the grid_ prefix and add Doxygen \brief plus inline dummy-argument docs throughout the module

API / compatibility notes

• mo_grid keeps the compatibility-facing API for data_t, mask_from_var, and data_from_var
• mo_grid_helper helper names were intentionally cleaned up and now use shorter names such as value_in_closed_interval, shift_longitude_near_query, quad_contains_point, append_candidate_id, and update_best_metric
• nearest_regridder_t is additive and does not change existing scaler_t behavior
• mo_grid_helper remains the implementation home for stateless helpers; mo_grid keeps grid semantics and TBPs

Tests

• extend test_mo_grid for exact nearest lookup, wide-grid behavior, spherical wrap handling, in_cell, helper TBPs, and spatial-index-backed grid lookup
• extend test_mo_grid_scaler for nearest regridding, mixed Cartesian/spherical cases, and derived target masks
• add test_mo_spatial_index
• add NetCDF fixtures for nearest-neighbor example/test coverage