[Website] Rewrite COEP/COOP headers as Document-Isolation-Policy in supporting browsers - #3028

Summary

This PR enables cross-origin isolation (required for SharedArrayBuffer) without breaking external embeds like YouTube videos.

Playground's client-side media processing experiment relies on SharedArrayBuffer, which requires cross-origin isolation. The traditional approach uses Cross-Origin-Embedder-Policy (COEP) and Cross-Origin-Opener-Policy (COOP) headers, but these are viral – once set, they affect the entire site and break external embeds that don't set matching headers.

Document-Isolation-Policy is a newer specification that provides cross-origin isolation on a per-document basis without these limitations. This PR feature-detects browser support for Document-Isolation-Policy and rewrites COEP/COOP headers to it automatically in supporting browsers.

Feature detection

This PR detects the Document-Isolation-Policy support by:

1. Loading a hidden iframe with the Document-Isolation-Policy header
2. Checking if SharedArrayBuffer is available in that iframe
3. Sending the result to the service worker
4. Rewriting headers on all subsequent scoped requests

As more browsers implement the specification, they'll automatically receive the better experience without code changes.

Handling cross-origin iframe restrictions

Document-Isolation-Policy introduces a side effect: the parent frame can no longer access iframe.contentWindow.location.href due to cross-origin restrictions. This would break Playground's URL bar synchronization.

To work around this, a WordPress MU plugin now posts the current URL to the parent frame on each page load. The boot script listens for these messages and updates the address bar accordingly, alongside the existing iframe load event handling.

Test plan

• Verify SharedArrayBuffer works in block editor with media processing enabled (Chrome 137+)
• Verify external embeds like YouTube still work in the block editor
• Verify URL bar updates correctly when navigating within WordPress
• Verify no regressions in browsers that don't support Document-Isolation-Policy

Here's a Blueprint that takes you directly to the post editor with the Gutenberg plugin installed and the media processing experiment enabled:

{
	"$schema": "https://playground.wordpress.net/blueprint-schema.json",
	"meta": {
		"title": "Enable all three Dataview Experiments in Gutenberg",
		"author": "bph",
		"description": "Blueprint example to enable multiple Experiments within the Gutenberg plugin ",
		"categories": ["Gutenberg", "Experiments"]
	},
	"landingPage": "/wp-admin/post-new.php",
	"plugins": ["gutenberg"],
	"login": true,
	"steps": [
		{
			"step": "runPHP",
			"code": "<?php require '/wordpress/wp-load.php'; update_option( 'gutenberg-experiments', array( 'gutenberg-media-processing' => true, 'gutenberg-new-posts-dashboard' => true, 'gutenberg-quick-edit-dataviews' => true )  );"
		}
	]
}

Related to #2954