ci: a new test file that never runs is worse than no test file

[myobie]

The adversarial delete matrix was added to tests/folder_sync.rs and no step was added here, so the two tests that reproduce a file-loss incident ran only on my laptop. CI stayed green throughout and told nobody.

That is the second time. #67 fixed the same shape for the binary's own tests: --lib plus a few named integration targets means a whole file can be invisible while the suite reports success.

Two changes

1. --test folder_sync now runs, so the guards against the 2026-08-25 loss are checked by something other than me remembering.
2. A step that fails when a tests/*.rs target is neither run nor named as a known gap. Adding a test file and nothing else now breaks the build. Forgetting should be loud.

I ran the guard both ways before shipping it

Against the current tree it passes and prints what it accounted for:

RUN: folder_sync
known gap: lifecycle
RUN: local_slice
known gap: pathwatch_slice
RUN: provisioning
known gap: shell
RUN: sync_slice
PASS - all accounted for

With a throwaway extra target dropped into tests/:

GUARD FIRES as intended: zz_probe_target

A guard that cannot fail is decoration.

The known gaps are named, not hidden

lifecycle, pathwatch_slice and shell still do not run here. All three pass on macOS locally; none is verified on Linux CI, so I am not switching them on in the same change that fixes the reporting. The list is meant to get shorter.

Agent: Silber.fabric