Effort-switch dialog gains an "ultracode" awareness check

The confirm-model-switch payload schema (Vbe) now validates modelKey and fromLevel/toLevel effort transitions, enforcing that toLevel is a downgrade from fromLevel. The related dialog code (K7) reads ultracode from app state and passes it into the effort-negotiation call (Net), and a new getLiveUltracode accessor joins getLiveEffort/getLiveModel. The nudge eligibility function ($be) also gained an ultracode parameter that, when truthy, unconditionally skips showing the nudge, alongside existing conditions like being mid-bridge-reattach or lacking userSettings write access.