publishStamps
internal fun publishStamps(dir: File, shards: List<Shard>, out: Map<Shard, File>, failed: Set<File>, wantEnv: List<String>): Set<String>
Record what this run produced; returns the file names worth keeping.
A shard whose source jar would not open is left out, so the next launch retries that one alone. The environment stamp is written last and only when its own shard came out whole: it is the token vouching for every other row, so stamping it over a gap would make that gap permanent.