Document-Isolation-Policy header scope is too narrow, breaking cross-window communication across admin screens #77633

Description

The Document-Isolation-Policy (DIP) header introduced in #75991 is only attached to a narrow set of block-editor page loads:

• `load-post.php`
• `load-post-new.php`
• `load-site-editor.php`
• `load-widgets.php`

Because DIP places each document into its own agent cluster, any admin screen that does not send the header ends up in a different agent cluster than the editor. Navigating between these screens (e.g., from the Site Editor to the Templates list, or from a pattern-edit popup back to its opener) breaks:

• Cross-window `postMessage` / `window.opener` communication used by preview popups and pattern editing.
• Agent-cluster continuity expected by SharedArrayBuffer-backed WASM media processing.

The workaround we've had to add to several E2E tests (`gutenberg-test-plugin-disable-client-side-media-processing`) is a symptom of the same issue — those specs were failing specifically because navigation crossed an agent-cluster boundary.

Steps to reproduce

1. On Chromium 137+, open the Site Editor.
2. Trigger any flow that relies on a cross-window handshake (e.g., pattern create-from-selection, template revert, preview popup, cross-admin navigation back to the editor).
3. Observe broken popup reuse, missing `window.opener`, or lost SharedArrayBuffer context.

Expected behavior

DIP should be sent consistently across all admin pages (subject to capability and feature checks), so admin navigations stay within a single agent cluster.

Proposed fix

Attach DIP via `admin_init` rather than the narrow `load-*.php` set, gate on `current_user_can( 'upload_files' )`, and preserve the existing skip for third-party page builders that override the block editor via an `action` query parameter. DIP remains admin-only — front-end previews are explicitly out of scope.

Fix implemented in #76662.

Environment

• Gutenberg: trunk (post-Media: Use Document-Isolation-Policy for cross-origin isolation on Chromium 137+ #75991)
• Browser: Chromium 137+
• Client-side media processing: enabled