Presentation Is a Guest
The conversation paints in one locked primary web view. Script talks only to the host bridge. The store is nonpersistent. Navigation is not a feature. This is not 'a web app in a shell.' A web app in a shell can wander, cache, and grow a second backend. A locked view can draw Markdown and cards the record already made durable.
Family chrome — a Sidekick — occupies a second isolated view when it is available. Different configuration, different origin, read-only host context, no engine pipe, no permission capability, no prompt submission. When that sibling surface is down, the coding client stays fully usable and the dock tells the truth. Engine runtime-zero is untouched: the engine still consults no one. The dock is optional client furniture.
What the Renderer Must Not Own
Elapsed time, permission cards, and review folds are projections of the record. A client reducer that 'helps' by keeping its own copy of the turn will drift the first time a child dies. The window recovers from the notebook, or it is lying. A renderer that caches the last pretty Markdown and replays it after a child restart is inventing a turn the record never wrote.