Move the five scene-state fields that drive per-model GPU upload + the
streaming residency loop into ViewportCore:
BufferPool pool_ — vertex+index sub-allocator
StreamingThread streaming_thread_ — background chunk reader
std::unordered_map<uint32_t, ModelGpuData> models_gpu_
— per-model state
uint32_t next_model_id_ — model-id allocator
uint32_t next_object_id_ — globally-unique object-id allocator
ViewportCore.h gains transitive includes for BufferPool / StreamingThread
/ ModelGpuData; ViewportWindow keeps the same names as reference aliases
so existing method bodies that touch them don't have to change.
Same risk profile as #84-a and #84-b: the storage moved but the values
are still set and consumed by the same code paths, so behaviour stays
identical.
Builds: desktop / bonsai / web all green. Tests 100/100.
Dion Moult
303f903a10