feat: put the retrobios app on a handheld card

--add-app, the result screen and a [y/N] question put the launcher of
muOS, KNULLI or ROCKNIX on the card the BIOS folder is on. The files come
from install/launchers/index.json at the same ref, each checked by size
and SHA-256 before the first is written, and the app event reports it.

scripts/generate_event_schema.py

@@ -45,7 +45,7 @@ KEY_TYPES: dict[str, dict] = {
         "pending", "code",
     ), _COUNT),
     **dict.fromkeys(("truncated", "ok", "interrupted"), _FLAG),
-    **dict.fromkeys(("systems", "other_drives", "dests"), _NAMES),
+    **dict.fromkeys(("systems", "other_drives", "dests", "written"), _NAMES),
     **dict.fromkeys(("platforms", "failed", "skipped", "left_out", "kept"), _RECORDS),
 }