Function default_layer_registry 

pub fn default_layer_registry() -> LayerRegistry

Default LayerRegistry for the server. Hint keys:

• "main_ai" → MainAIMiddleware (= fires SpawnHook before/after)
• "senior_escalation" → SeniorEscalationMiddleware (= on ok=false, escalates via SeniorBridge.ask)
• "operator_delegate" → OperatorDelegateMiddleware (= when an operator backend is registered, delegates the entire spawn)

Including any of these keys in Blueprint.spawner_hints.layers causes them to be wrapped into a SpawnerStack at service::linker::link time (= per-launch; the old engine.bind global-state path is retired). Callers (the engine builder side) receive it via Engine::new_with_layers(cfg, mse_server::default_layer_registry()).