Use an in-memory store for local single-rank CPU startup; distributed, external, elastic and existing groups retain their previous initialization. Verify a real subgroup collective and negative cases. The macOS smoke uses the same local rendezvous before compilation. Co-authored-by: OpenAI Codex <noreply@openai.com> Signed-off-by: yangzhuxinyzx <153831768+yangzhuxinyzx@users.noreply.github.com>