gitignore: mame scratch dirs written into the repo root (cfg/, nvram/, roms/)

.gitignore

@@ -38,6 +38,11 @@ tools/ghidra-headless-mcp/
 harness/mame/board-*.log
 harness/mame/gui.log
 
+# MAME scratch written into the cwd when it is run from the repo root
+/cfg/
+/nvram/
+/roms/
+
 # Large media: the source recording stays out of git (a YouTube download,
 # trivially re-obtainable).  The derived frames and contact sheets ARE committed,
 # as evidence — see reference/vt1200-boot-video/NOTES.md for the regeneration recipe.