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.

Docs/build.md

@@ -46,9 +46,15 @@ Engine/Build/BatchFiles/RunUAT.sh BuildCookRun \
   -archive -archivedirectory="/run/media/system/DATA/Builds/GigLine" \
   -unattended
 # Expected: "BUILD SUCCESSFUL" (~1 min for the blank project; archive ~1.1 GiB)
-# Launch test: /run/media/system/DATA/Builds/GigLine/Linux/GigLine.sh
+# Launch test (plain, no -map): /run/media/system/DATA/Builds/GigLine/Linux/GigLine.sh
 ```
 
+**Launch test must open the DemoMap.** A plain `./GigLine.sh` should load
+`/Game/Hollywood/DemoMap` (set via `DefaultEngine.ini` `GameMapsSettings`). If it
+instead opens `/Engine/Maps/Templates/OpenWorld`, the game shows an empty
+non-interactive "stuck" scene — check that `GameDefaultMap` is in
+`DefaultEngine.ini`, not `DefaultGame.ini`. Confirm the loaded map in the engine
+log: `.../Linux/GigLine/Saved/Logs/GigLine.log` (`LoadMap: /Game/Hollywood/DemoMap`).
 The archive directory is deliberately **outside** the repository; never archive into the checkout.
 
 ## Headless content checks