SelectorTarget

What a selector composable can be targeted at. Callers can pass:

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