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

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).

Tools/ue_build_demo_map.py

@@ -301,6 +301,19 @@ def build():
         mesh = unreal.EditorAssetLibrary.load_asset(f"/Game/Hollywood/Site/{name}")
         if mesh is None:
             continue
+        # The imported terrain carries a bounding-box simple collision proxy,
+        # which puts an invisible lid ~8 m over the whole block: the player
+        # walks on that box instead of the street. Force trace against the real
+        # triangles so the carved street corridor is actually open.
+        if "Terrain" in name:
+            asset = unreal.EditorAssetLibrary.load_asset(f"/Game/Hollywood/Site/{name}")
+            if asset:
+                unreal.GigLineAssetTools.set_complex_collision_as_simple(asset, True)
+                # The flag lives on the asset, not the map: persist it or the
+                # next session reloads the bbox proxy and the street is buried.
+                saved_asset = unreal.EditorAssetLibrary.save_loaded_asset(asset)
+                log(f"terrain collision: {unreal.GigLineAssetTools.get_collision_complexity(asset)} "
+                    f"(saved asset: {saved_asset})")
         actor = spawn(unreal.StaticMeshActor, unreal.Vector(0, 0, 0), f"Demo_{name}")
         if actor:
             comp = actor.static_mesh_component
@@ -372,14 +385,17 @@ def build():
     # sidewalk is at Y -1912..-1053 and the SOUTH sidewalk at Y +718..+1447.
     # The hero facade is on the SOUTH side at Y +1307. Stand on the north
     # sidewalk and face south (+Y, yaw 90) across the road at it.
-    # Z: the terrain here is ~Z 99-138. The capsule half-height is 87.5 cm, so
-    # feet must start ABOVE the surface or the point is embedded and UE rejects
-    # the PlayerStart ("NO PLAYERSTART with positive rating"), dropping the pawn
-    # at the world origin to fall through. Cap feet ~80 cm above grade: centre
-    # ~138+80+87 = 305, so Z 300 lets it drop cleanly and gravity settles it.
-    spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1700.0, 300.0), "Demo_PlayerStart",
+    # CRITICAL: the terrain is only carved inside the street corridor, and
+    # terrain_from_dem.py --corridor 10.7 is a HALF-width around the road
+    # centreline (Y ~= -167), so the cut band is Y -1237..+902. A PlayerStart at
+    # Y -1700 sits OUTSIDE it, under ~8 m of uncut terrain: the capsule spawns
+    # embedded, depenetration pops it to Z ~909 (terrain top 819 + half-height
+    # 87.5) and it never touches the sidewalk. Y -1150 is on the north sidewalk
+    # (Y -1912..-1053) AND inside the cut, so the pavement is the ground.
+    # Z 600 is clear air; the capsule drops onto the pavement and settles.
+    spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1150.0, 600.0), "Demo_PlayerStart",
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))
-    log("staged: Demo_PlayerStart on the north sidewalk (cap above grade), facing south (+Y)")
+    log("staged: Demo_PlayerStart on the cut north sidewalk, facing south (+Y)")
 
     # A lockable LFS .umap checks out read-only, which makes save_map silently
     # return False — chmod u+w first or the staged actors never persist.