Function event_layer 

pub fn event_layer(event: &FabricEvent) -> &'static str

Classify a FabricEvent into its originating layer.

Returns "fabricbios" for protocol-level events (leases, teardown, auth, replay, token validation, security profile, ops) and "grafos" for graph/runtime-level events (services, tasklets, rewrites, messages, objects, admission, placement, preemption, quota, token minting, listeners, sessions).