UI extensibility guide, for every renderer

Two new documentation pages explain how to extend the platform's UI: which of the three lanes a feature belongs in (content plugin, view pack, or core), the seams each one plugs into, and how to roll an extension out to deployments. A companion page covers writing views that render on the web portal, the Next.js shell, and the mobile app alike — what inherits automatically, the parity checklists, and the definition of done for a new control. The new /ui-extensibility skill gives agents the same guidance in procedure form.