Add extensible content parser with Markpub support

.github/changelog/content-parser

@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add rich content support for standard.site documents using the Markpub format.

composer.json

@@ -27,8 +27,7 @@
 	},
 	"autoload": {
 		"classmap": [
-			"includes/",
-			"integrations/"
+			"includes/"
 		]
 	},
 	"scripts": {

includes/class-atmosphere.php

@@ -13,7 +13,6 @@
 use Atmosphere\Transformer\Document;
 use Atmosphere\Transformer\Publication;
 use Atmosphere\Transformer\TID;
-use Atmosphere\Integrations\Load;
 use Atmosphere\WP_Admin\Admin;
 
 /**
@@ -41,8 +40,8 @@ public function init(): void {
 		\add_action( 'init', array( $this, 'register_wellknown_rewrite' ) );
 		\add_action( 'template_redirect', array( $this, 'serve_wellknown_publication' ) );
 
-		// Plugin integrations.
-		Load::init();
+		// JSON preview for AT Protocol records.
+		\add_action( 'template_redirect', array( $this, 'preview' ) );
 
 		// Post lifecycle hooks.
 		\add_action( 'transition_post_status', array( $this, 'on_status_change' ), 10, 3 );
@@ -145,6 +144,45 @@ public function serve_wellknown_publication(): void {
 		exit;
 	}
 
+	/**
+	 * Serve a JSON preview of the AT Protocol record for a post.
+	 *
+	 * Append ?atproto to a singular post URL to see the document
+	 * record JSON. Optionally pass ?atproto={parser} to preview
+	 * with a specific content parser (requires the parser to be
+	 * registered via the atmosphere_content_parser filter).
+	 */
+	public function preview(): void {
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
+		if ( ! isset( $_GET['atproto'] ) || ! \is_singular() ) {
+			return;
+		}
+
+		if ( ! \current_user_can( 'edit_posts' ) ) {
+			return;
+		}
+
+		$post = \get_queried_object();
+
+		if ( ! $post instanceof \WP_Post ) {
+			return;
+		}
+
+		if ( ! \in_array( $post->post_type, Backfill::syncable_post_types(), true ) ) {
+			\status_header( 404 );
+			exit;
+		}
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		\status_header( 200 );
+		\header( 'Content-Type: application/json; charset=utf-8' );
+		// phpcs:ignore WordPress.WP.AlternativeFunctions.json_encode_json_encode
+		echo \wp_json_encode( $record, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE );
+		exit;
+	}
+
 	/**
 	 * Handle post status transitions.
 	 *