feat: emit parked-car data as a runtime-loadable asset

[nigel-hall-codes]

Decouple parked cars from the baked chunk prefab so they can be view-gated (enabling change for #202).

Scope

• Stop baking parked-car prefab instances into the chunk prefab (SFMapImporterWindow.ImportParkedCars).
• Emit the existing chunk_CC_RR_parked.json payload (position, rotation, model index, street) as a runtime-loadable asset per chunk.
• Keep a flag/path to fall back to baked cars so the change is reversible while the streamer (next issue) is in flight.

Blocked by the measurement spike (GO only). Parent: #202.