Real-time collaboration: add new REST endpoints, setting, and registered post meta.

Syncs/merges the PHP changes from the Gutenberg PR WordPress/gutenberg#75366.

In Gutenberg, we have added support for real-time collaboration using CRDT documents (via the [Yjs library](https://yjs.dev/)). This work has suggested the following additions to WordPress:

1. A default "sync provider" based on HTTP polling that allows collaborators to share updates with each other. Previously, we relied on WebRTC connections between collaborators for this purpose, but it proved unreliable under many network conditions.
   - Our solution is designed to work on any WordPress installation. 
   - HTTP polling is the transport we identified as most likely to work universally.
   - Given the isolation and lifecycle of PHP processes, updates must be stored centrally in order to be shared among peers. We have chosen to store updates in post meta against a special post type, but alternate storage mechanisms are possible.
   - Collaborative editing can involve syncing multiple CRDT documents. To limit the number of connections consumed by this provider, requests are batched.
   - To prevent unbounded linear growth, updates are periodically compacted.
   - To avoid excessive load on lower-resourced hosts, this provider will benefit from usage limits (e.g., a maximum of three connected collaborators) enforced by the client (Gutenberg).

2. A new registered post meta that allows Gutenberg to persist CRDT documents alongside posts.
   - This provides all collaborators with a "shared starting point" for the collaborative session, which avoids duplicate updates.
   - Content stored in the WordPress database always remains the source of truth. If the content differs from the persisted CRDT document, the CRDT document is updated to match the database.

3. A new Writing setting that allows users to opt-in to real-time collaboration.
   - Enabling real-time collaboration disables post lock functionality and connects users to the sync provider.

4. A behavior change to autosaves is needed. When the the original author is editing a draft post (post_status == 'draft' OR 'auto-draft') and they hold the post lock, the autosave targets the actual post instead of an autosave revision. This puts the post data and the persisted CRDT document out of sync and leads to duplicate updates. When real-time collaboration is enabled, all collaborators must autosave in the same way.

This PR provides a proposed implementation of the changes above. This corresponding Gutenberg PR moves the work from the `experimental` directory to `lib/compat`:

WordPress/gutenberg#75366

Cumulative work to add this functionality can be found using this label:

https://github.com/WordPress/gutenberg/issues?q=label%3A%22%5BFeature%5D%20Real-time%20Collaboration%22%20is%3Apr

Developed #10894.

Props czarate, paulkevan, ellatrix, timothyblynjacobs, westonruter, jorgefilipecosta, mindctrl.
Fixes #64622.


git-svn-id: https://develop.svn.wordpress.org/trunk@61689 602fd350-edb4-49c9-b593-d223f7449a82

Author: ellatrix

src/wp-admin/includes/schema.php

@@ -563,6 +563,9 @@ function populate_options( array $options = array() ) {
 
 		// 6.9.0
 		'wp_notes_notify'                 => 1,
+
+		// 7.0.0
+		'enable_real_time_collaboration'  => 1,
 	);
 
 	// 3.3.0

src/wp-admin/options-writing.php

@@ -109,6 +109,13 @@
 	</select>
 </td>
 </tr>
+<tr>
+<th scope="row"><label for="enable_real_time_collaboration"><?php _e( 'Collaboration' ); ?></label></th>
+<td>
+	<input name="enable_real_time_collaboration" type="checkbox" id="enable_real_time_collaboration" value="1" <?php checked( '1', get_option( 'enable_real_time_collaboration' ) ); ?> />
+	<label for="enable_real_time_collaboration"><?php _e( 'Enable real-time collaboration' ); ?></label>
+</td>
+</tr>
 <?php
 if ( get_option( 'link_manager_enabled' ) ) :
 	?>

src/wp-admin/options.php

@@ -153,6 +153,7 @@
 		'default_email_category',
 		'default_link_category',
 		'default_post_format',
+		'enable_real_time_collaboration',
 	),
 );
 $allowed_options['misc']    = array();

src/wp-includes/certificates/ca-bundle.crt

@@ -1,7 +1,7 @@
 ##
 ## Bundle of CA Root Certificates
 ##
-## Certificate data from Mozilla as of: Tue Dec  2 04:12:02 2025 GMT
+## Certificate data from Mozilla as of: Tue Nov  4 04:12:02 2025 GMT
 ##
 ## Find updated versions here: https://curl.se/docs/caextract.html
 ##
@@ -15,8 +15,8 @@
 ## an Apache+mod_ssl webserver for SSL client authentication.
 ## Just configure this file as the SSLCACertificateFile.
 ##
-## Conversion done with mk-ca-bundle.pl version 1.30.
-## SHA256: a903b3cd05231e39332515ef7ebe37e697262f39515a52015c23c62805b73cd0
+## Conversion done with mk-ca-bundle.pl version 1.29.
+## SHA256: 039132bff5179ce57cec5803ba59fe37abe6d0297aeb538c5af27847f0702517
 ##
 
 
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 -----END CERTIFICATE-----
 
+CommScope Public Trust ECC Root-01
+==================================
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+CommScope Public Trust ECC Root-02
+==================================
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+CommScope Public Trust RSA Root-01
+==================================
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+
+CommScope Public Trust RSA Root-02
+==================================
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 Telekom Security TLS ECC Root 2020
 ==================================
 -----BEGIN CERTIFICATE-----

src/wp-includes/collaboration.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * Bootstraps collaborative editing.
+ *
+ * @package WordPress
+ * @since 7.0.0
+ */
+
+/**
+ * Injects the real-time collaboration setting into a global variable.
+ *
+ * @since 7.0.0
+ *
+ * @access private
+ */
+function wp_collaboration_inject_setting() {
+	if ( get_option( 'enable_real_time_collaboration' ) ) {
+		wp_add_inline_script(
+			'wp-core-data',
+			'window._wpCollaborationEnabled = true;',
+			'after'
+		);
+	}
+}