SelectorTarget

What a selector hook can be targeted at. Callers can pass any of:

• undefined — root namespace (cheap: served by the always-on root store)
• a SubagentDiscoverySnapshot — the snapshot returned by stream.subagents.get(...)
• a SubgraphDiscoverySnapshot — the snapshot returned by stream.subgraphs.get(...)
• an explicit { namespace: string[] } — any other namespaced scope
• a raw string[] — escape hatch identical to the object form