v0.69.0

Since 0.69.0 every server this suite spawns starts its engine download at
main(). The fast selection kept downloads away with a throwaway cache and a
one-second deadline, and on the ubuntu runner of the merge commit a complete
firefox-34 tree appeared in that cache anyway: the archive arrived inside the
second. The guard went red for the right reason, and main with it.

The prohibition is the core's own word for "there is nothing to download": a
LOCAL seal, one with no assets, on which ensure_binary refuses before it
touches the network or the cache. The conftest derives it from the packaged
seal - tag, version and Playwright range unchanged, the top-level build id
the host leg's, so a real binary named by STEALTHFOX_BINARY still verifies -
and points INVISIBLE_SEAL_FILE at it, inherited by every child process. It
does so without importing the core, because the light jobs install pytest
alone. The guard now asks ensure_binary and expects the refusal, instead of
reading the environment and hoping the network is slow.