Trusted native plugin slot for the content editor SEO body

[logelog]

Problem

Trusted native plugins can add full admin pages, but they cannot enhance the existing SEO section in the content editor without adding a second SEO panel or relying on DOM injection. A second panel duplicates native fields and fragments the editing workflow.

Proposed extension point

Add one narrowly scoped trusted-native contribution: contentEditorSeoSlots.

A matching contribution may replace only the body of EmDash's existing SEO section. EmDash continues to own:

• the SEO heading and placement;
• editor layout and accessibility;
• collection and role filtering;
• plugin enable/disable lifecycle;
• deterministic selection;
• error isolation and the native fallback.

The native SEO body is rendered when no contribution applies, when a plugin is disabled or stale, when its predicate fails, or when its component throws. This keeps the editor usable and avoids plugin-owned page structure.

The slot is limited to trusted React admin plugins. Sandboxed plugins keep their existing Block Kit boundary.

Scope

This proposal does not add movable panels, content-list columns, sorting, or arbitrary named slots. Those are separate extension points and can be reviewed independently.

Validation

The focused implementation includes resolver coverage for lifecycle, filtering, duplicate ids, malformed exports, and predicate failures, plus editor integration coverage for replacement and native fallback behavior. Admin and root typechecks, lint, formatting, and targeted browser tests pass.

The branch will be linked from the focused PR.