qa: make the demo walkable — terrain collision root cause, spawn, scripted QA

The reported fall-through was a misdiagnosis: the player never fell. The
imported Terrain_DEM had only a bounding-box simple collision proxy, so a
downward trace anywhere in the block hit the terrain at Z 819 (the mesh
bbox max) and the street sat under an invisible 8 m lid. Standing on that
box looked plausible, which is why it read as working.

- add UGigLineAssetTools (C++): force CTF_UseComplexAsSimple on a static
  mesh. UE 5.8 dropped bUseComplexAsSimpleCollision and Python cannot set
  the flag; the builder also saves the asset, since the flag lives there
  and a map-only save silently reverts it.
- spawn (-13336, -1150, 600): inside the carved corridor (the cut is a
  10.7 m half-width around the road centreline), lands on the road at 456.
- fix ResetToStart returning to the mid-air PlayerStart captured in
  BeginPlay: StartLocation is now captured on the first grounded frame.
- HighResShot 1280x720 instead of "1280 720", which is a multiplier.
- add -qaselftest: scripted walk/reset/pause on a core ticker plus a
  swept ground transect. Results: walk 746 cm/2 s with Z steady, reset
  0 cm from spawn, pause/unpause 1/0, transect 84 samples 0 holes.
- rebase onto main, keeping the frontage materials and placement fixes.

Game/GigLine/Source/GigLine/GigLineGameModeBase.h

@@ -18,4 +18,8 @@ class GIGLINE_API AGigLineGameModeBase : public AGameModeBase
 
 public:
 	AGigLineGameModeBase();
+
+protected:
+	/** Logs whether a local player connected to a pawn with input (QA). */
+	virtual void HandleStartingNewPlayer_Implementation(class APlayerController* NewPlayer) override;
 };