Sharing and permissions

## Goal

Let users control who can see or access Cortext pages and collection views.

## Scope

* Add private, shared, and public states for Cortext content.
* Add share controls for pages and collection views.
* Reuse WordPress capabilities where possible.
* Coordinate with frontend rendering so public content renders without leaking private rows or pages.
* Keep the first version focused on access control and sharing UX.

Start by mapping the existing page and collection-view rendering and the WordPress capabilities used for access checks. Define how private, shared, and public states and their share controls fit together, then verify that public rendering cannot expose private rows or pages. No files or tests are named, so the implementation entry points and completion checks must first be established.

Written by the indexing model from the issue text.