Trusted native content editor extension panels

[logelog]

Native plugins can add complete admin pages today, but they cannot add contextual UI to the content list or editor without duplicating core panels or relying on DOM injection. That makes focused editorial plugins difficult to integrate safely and consistently.

I propose a trusted-native admin extension contract with three host-owned surfaces:

• typed content-list columns;
• movable content-editor panels in the main or sidebar region;
• named native slots, beginning with the core SEO panel body.

The host remains responsible for role/collection filtering, disabled-plugin behavior, layout, persistence, error isolation, and native fallback. For named slots, plugins can append to or replace only the slot body; the first applicable replacement wins deterministically, and the native body returns automatically if the plugin is absent, disabled, inapplicable, or throws.

Panel ordering is stored per user and collection by the host. Pointer drag handles are paired with keyboard-accessible move actions. Named slots stay outside movable layout so they do not duplicate core headings or own page structure.

A tested implementation is available in PR #2104:
#2104

The implementation is split into reviewable commits and includes focused selector, rendering, fallback, persistence, and interaction coverage.