Compact code grid code

[vcharraut]

What

Cleanup-only refactor of the grid-map code in drive.h plus a small
behavioral fix to how the grid's spatial bounds are computed. No change to
the training loop or Python side.

Changes

GridMap struct (drive.h)

• Drop unused cell_size_x / cell_size_y fields (always equal to
  GRID_CELL_SIZE).
• Regroup fields by kind (scalars, count arrays, entity arrays) and remove
  stale inline comments.

init_grid_map

• Restrict the grid's bounding box to lanes and road edges via the new is_road_grid_candidate() helper (datatypes.h), instead of all road elements. Road lines no longer stretch the grid extent.
• Use an element pointer alias in the count/populate passes.
• Merge the two INVALID_POSITION guards into one.
• Remove the dead cell_entities_insert_index != 0 re-zeroing loop
  (the array comes straight from calloc).

apply_infraction_behavior() helper

• Extract the repeated STOP_AGENT / REMOVE_AGENT block (offroad,
  collision, red-light) into one inline helper in compute_metrics.

Misc

• init_neighbor_offsets, cache_neighbor_offsets,
  get_neighbors_entities, get_grid_index: shorter local names
  (vr, rel_x, cell_x/cell_y, cols) and dropped redundant comments.

Net

+72 / −128 across drive.h and datatypes.h.

[Copilot]

Pull request overview

This PR refactors and “compacts” the Drive grid-map code in drive.h, mainly by reorganizing GridMap fields, simplifying several grid/neighbor helper routines, and deduplicating infraction behavior handling.

Changes:

• Reordered/trimmed GridMap struct members (including removing unused cell_size_x/y fields).
• Added apply_infraction_behavior() to centralize STOP/REMOVE handling for offroad/collision/red-light infractions.
• Simplified variable naming and small logic cleanup in grid index calculation, grid map initialization, and neighbor offset caching.

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