freezeQueue took a second prState read per unverified slot and filed anything
but an exact MERGED under unmerged, so a slot that read MERGED at stopped()
and was unreadable a moment later was recorded as unmerged (the record half of
train 24's #3453 misfile). stopped() now reads each unverified slot once and
passes those states in; a MERGED slot stays merged, and a slot unreadable
beside a merged one is kept out of unmerged and named in the reason. The
fixture's gh shim gains a per-PR vanish count so a test can change GitHub's
answer between two reads.
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>