The lab's check ran in the TypeScript toolchain, which holds no Go compiler,
so the replays register was never compiled before a merge. merge-check.sh now
declares replays/merge-check.sh as its Go part, which the build seat runs in
the Go toolchain: gofmt, go vet and the register's own tests. Both new
replays are proved: each fails on the commit before its fix and passes on it.