ReadonlyadapterReadonlybridgeReadonlybuiltinsApplets the host ships with itself.
ReadonlyfetchReadonlygrantsReadonlyhandlesLive handles keyed by window id; shared with the bridge's revocation hook.
ReadonlyhostReadonlyinstalledEvery planned applet is installed, so others can hand it work.
ReadonlyopenReadonlypipelineReadonlypoolReadonlysubscriptionsReadonlytiers
What the applet lifecycle needs from the composed shell.