docs: note plain launch must open DemoMap (stuck-scene trap)

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