test(profile): the road is a flat slab at 366 while the sidewalks slope

Added a -profile probe: one downward trace every 20 m along the street at the player's Y, naming
the surface each time. It settles the question the bounding boxes could not:

  x=-14336..-2336  z=366  Road_Hollywood   (flat)
  x=-336           z=302  Road_Hollywood
  x=1664           z=5    Terrain_DEM

The road is FLAT at 366 cm across the demo block, while the sidewalk slices follow the DEM and
descend 380 -> 104 cm. So the ~2 m gap is not an import artefact and not a collision problem: the
road itself does not follow the elevation the sidewalks follow.

That also explains why every sidewalk fix failed - the surface the player stands on is this flat
road, and the sloped sidewalk is metres below it.

Next: how rows is built in Art/blender/street/street_mesh.py. The road and the sidewalk read the same
dem_z(x, y), so a flat road means the sampler returns the same value for every row.

Game/GigLine/Source/GigLine/GigLineCharacter.cpp

@@ -1045,6 +1045,36 @@ void AGigLineCharacter::Tick(float DeltaSeconds)
 		if (FootProbeDelay <= 0.f)
 		{
 			LogFootProbes();
+			if (FParse::Param(FCommandLine::Get(), TEXT("profile")) && GetWorld())
+			{
+				// Ground profile along the street: one downward trace every 20 m at the
+				// player's Y, naming the surface each time. Comparing the road's profile
+				// with the sidewalks' is what the per-mesh bounding boxes could not do.
+				const FVector Base = GetActorLocation();
+				for (int32 Step = 0; Step < 14; ++Step)
+				{
+					const float X = Base.X + Step * 2000.0f - 1000.0f;
+					FHitResult Hit;
+					FCollisionQueryParams Q(TEXT("GigLineProfile"), false, this);
+					const bool bHit = GetWorld()->LineTraceSingleByChannel(Hit,
+						FVector(X, Base.Y, Base.Z + 2000.0f),
+						FVector(X, Base.Y, Base.Z - 4000.0f), ECC_WorldStatic, Q);
+					FString MeshName = TEXT("none");
+					if (bHit)
+					{
+						if (const UStaticMeshComponent* HitMesh =
+								Cast<UStaticMeshComponent>(Hit.GetComponent()))
+						{
+							if (const UStaticMesh* Asset = HitMesh->GetStaticMesh())
+							{
+								MeshName = Asset->GetName();
+							}
+						}
+					}
+					UE_LOG(LogTemp, Warning, TEXT("GigLine profile x=%.0f z=%.0f mesh=%s"),
+						X, bHit ? Hit.ImpactPoint.Z : -9999.f, *MeshName);
+				}
+			}
 			if (bProbeAndQuit && GetWorld())
 			{
 				// One frame later, so the log lines are flushed before the process exits.