SelectorTarget

What a selector primitive 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().get(...));
• a SubgraphDiscoverySnapshot (stream.subgraphs().get(...));
• an explicit { namespace: string[] };
• a raw string[] escape hatch.

Selectors also accept a Signal<SelectorTarget> so callers can feed a computed(() => ...) and have the projection rebind automatically.