docs: design for shared-iframe docks with soft navigation

Add a design proposal for hosting a foreign multi-tab devtool (e.g. Nuxt
DevTools) as first-class hub docks that share one live iframe and switch
views via client-side soft navigation.

Splits the design into a hub-owned data model (a frameId shared-iframe axis
orthogonal to groupId, plus client-only dock registration) and a versioned,
origin-locked host<->iframe postMessage protocol for the live nav loop. The
embedded app ships only a small postMessage shim and takes no hub/RPC
dependency. Includes the viewer UI-behavior contract for Vite DevTools, the
adapter behavior, fallback semantics, message tables, sequences, and TS
type sketches.

Author: antfubot