Generic seam only — no board, OS or runtime specifics in the OSS tree:
an overlay may register an engine that decides, per board, whether a run
needs the Linux guest or can happen in the browser. The decision is data
(engine + reason + where) so the UI can explain a 90 s boot instead of
just taking it, and BoardInstance carries engineMode / enginePinned so
the user's choice is predictable and the terminal panel knows whether an
interactive shell exists. Nothing registers in OSS: the QEMU path is
untouched.
David Montero Crespo