dwt: 50%-pattern real-unit capture at the correct 1280x1024/256-stride geometry

dwt/notes/REAL_UNIT.md

@@ -155,3 +155,11 @@ python3 dwt/screenshot.py --host 100.103.100.103 --real --out screen.png
 the Icon Box resize buttons and the `Ethernet Port Error - 24` box.
 
 The emulator geometry is unchanged: 1024x1024 with a 128-byte stride.
+
+### Background references
+
+Both backgrounds are captured at the correct geometry:
+`dwt/evidence/real-unit-1280-1to1.png` (100% white root) and
+`dwt/evidence/real-unit-1280-50pct.png` (50% `1010` root, the owner's
+preference). Only the root's image bytes change; the 256-byte stride and the
+1280x1024 size are identical.