chore: import with bakeParkedCars off; rely on runtime ParkedCarStreamer

[nigel-hall-codes]

Parent: #259 (techdesign) — see sdlc/#259/design.md, lever L2.
Reanalyzed 2026-07-04: unblocked — #208/#209/#210 are all closed; scope is now trivial.

Why

SFMapImporterWindow still defaults bakeParkedCars = true (SFMapImporterWindow.cs:43), instantiating every parked car into the chunk prefab — import time plus fatter SaveAsPrefabAsset on every chunk. The runtime replacement has shipped: ParkedCarStreamer (#210) streams pooled, frustum-gated cars from the _parked.json TextAssets the importer already copies (ImportParkedCarsJson), and the spike (#208) and data asset (#209) are done.

What (reduced scope)

1. Flip the importer default to bakeParkedCars = false; keep the toggle for debugging.
2. Verify parity on a real chunk: streamer-spawned cars match previously-baked placements (position/rotation/model/scale, incl. slope conform from fix: parked cars on slopes are placed level, clipping into the road #228).
3. Confirm the scene actually has a ParkedCarStreamer (the "Add Parked Car Streamer" menu item) so cars don't silently vanish from the default flow.

Done when

Default imports bake no parked cars into prefabs, the streamer renders them equivalently in play mode, and per-chunk parked cars + save prefab timings drop (per #446 instrumentation).