docs(termtris): build the harness binary before reproducing evidence

apps/termtris/VT1200.md

@@ -94,6 +94,16 @@ codes (`0x9b` + `A`/`B`/`C`/`D`), which `game_input` translates back to
 
 ## Running it in MAME
 
+The emulator must be built from the driver in this tree; an older
+`/home/arti/code/k-space/mame0289/vt1200` may still carry the pre-VR319
+geometry:
+
+```sh
+harness/mame/build.sh          # rebuild the vt1200 subtarget (JOBS=4 on small hosts)
+```
+
+Then:
+
 ```sh
 apps/termtris/tests/play.sh          # load, click, snapshot
 apps/termtris/tests/play_keys.sh     # move / panels / quit acceptance runs