SelectorTarget

What a selector composable targets. Callers can pass:

• undefined / null — root namespace (served by the always-on root store; no extra subscription opens);
• a SubagentDiscoverySnapshot (stream.subagents.get(...));
• a SubgraphDiscoverySnapshot (stream.subgraphs.get(...));
• an explicit { namespace: string[] };
• a raw string[] escape hatch.