macOS caps shm_open names at PSHMNAMLEN (31 bytes including the leading
slash) and answers ENAMETOOLONG beyond that, while Linux allows 255. The
dispatcher tests name segments oak-procpool-ut-<pid>-<label>, so every
procpool test failed on macOS as soon as the earlier crash stopped
masking them. Fold longer keys into a deterministic FNV-1a short name in
one place; owner, worker and unlink all derive the name through it, so
they keep meeting on the same segment.