Two new methods on GpuWorldState, used by two-tier streaming (T13):
- RemoveEntitiesFromLandblock(id): drop all entities from an LB while
keeping the terrain. Used for Near->Far demote (player walks past the
inner ring; LB stays loaded but entities leave).
- AddEntitiesToExistingLandblock(id, entities): merge new entities into
an already-loaded LB record. Used for Far->Near promote (terrain is
already on the GPU; just streaming the entity layer in).
Falls back to the pending bucket if the LB hasn't loaded yet.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
