fix(collision): terrain bbox proxy buried the whole street

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The player never fell through - it stood on an invisible 8 m box. The
imported Terrain_DEM carried only a bounding-box simple collision proxy,
so every downward trace in the block hit Demo_Terrain_DEM at Z 819 (the
mesh bbox max) and the road/sidewalk underneath were unreachable. The
view from up there looked plausible, which is why it read as working.

- add UGigLineAssetTools (C++): set complex-as-simple collision on a
  static mesh; UE 5.8 exposes ECollisionTraceFlag, not
  bUseComplexAsSimpleCollision, and Python cannot set it at all.
- builder forces + saves the terrain asset as complex-as-simple (the flag
  lives on the asset, so saving only the .umap loses it).
- spawn (-13336, -1150, 600): inside the carved corridor; lands on the
  road at Z 456 (feet 368 on road top 366).
- HighResShot 1280x720, not "1280 720" (that is multiplier 1280).
- keep SIM + single ground-probe diagnostics (TEMP QA).

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