Fixes #878: extend KDTree path to allocation/direction to prevent OOM (#893)

The memory-safe KDTree path was previously gated to PROXIMITY mode only.
This extends it to handle ALLOCATION and DIRECTION modes, since
cKDTree.query() returns both distances and nearest-target indices —
enough to derive allocation values and direction angles.

Changes:
- Add _vectorized_calc_direction() for array-based compass directions
- Extend _stream_target_counts() to cache target pixel values
- Extend _collect_region_targets() to return target values alongside coords
- Extend _kdtree_chunk_fn() and _tiled_chunk_query() for all three modes
- Thread process_mode and values_cache through the full call chain
- Remove process_mode == PROXIMITY restriction from KDTree gate
- Add memory guard for non-KDTree fallback (GREAT_CIRCLE, no scipy)
- Update memory estimate from 48 to 52 bytes per target

Author: brendancol