feat: add json file transport

src/Transport/FileTransportInterface.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * File package transport boundary.
+ *
+ * @package WPContentSync
+ */
+
+namespace WPContentSync\Transport;
+
+use WPContentSync\Package\ContentPackage;
+
+interface FileTransportInterface {
+	public function export( ContentPackage $package ): string;
+
+	public function import( string $contents ): ContentPackage;
+}

src/Transport/JsonFileTransport.php

@@ -0,0 +1,45 @@
+<?php
+/**
+ * JSON file transport implementation.
+ *
+ * @package WPContentSync
+ */
+
+namespace WPContentSync\Transport;
+
+use WPContentSync\Package\ContentPackage;
+use WPContentSync\Package\PackageValidator;
+
+final class JsonFileTransport implements FileTransportInterface {
+	private PackageValidator $validator;
+
+	public function __construct( PackageValidator $validator ) {
+		$this->validator = $validator;
+	}
+
+	public function export( ContentPackage $package ): string {
+		$json = wp_json_encode( $package->toArray(), JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES );
+
+		if ( false === $json ) {
+			throw new \RuntimeException( 'Unable to encode content package JSON.' );
+		}
+
+		return $json;
+	}
+
+	public function import( string $contents ): ContentPackage {
+		$decoded = json_decode( $contents, true );
+
+		if ( ! is_array( $decoded ) ) {
+			throw new \InvalidArgumentException( 'The selected file is not valid JSON.' );
+		}
+
+		$result = $this->validator->validate( $decoded );
+
+		if ( ! $result->isValid() ) {
+			throw new \InvalidArgumentException( implode( ' ', $result->errors() ) );
+		}
+
+		return ContentPackage::fromArray( $decoded );
+	}
+}

tests/Unit/Transport/JsonFileTransportTest.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace WPContentSync\Tests\Unit\Transport;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Package\ContentPackage;
+use WPContentSync\Package\PackageChecksum;
+use WPContentSync\Package\PackageValidator;
+use WPContentSync\Transport\JsonFileTransport;
+
+class JsonFileTransportTest extends TestCase {
+	public function test_it_exports_pretty_json_packages(): void {
+		$transport = new JsonFileTransport( new PackageValidator() );
+		$json      = $transport->export( $this->package() );
+
+		self::assertStringContainsString( "\n", $json );
+		self::assertStringContainsString( '"schema_version": "1.0"', $json );
+	}
+
+	public function test_it_imports_valid_json_packages(): void {
+		$transport = new JsonFileTransport( new PackageValidator() );
+		$package   = $transport->import( $transport->export( $this->package() ) );
+
+		self::assertSame( '1.0', $package->schemaVersion() );
+		self::assertSame( 'https://example.test', $package->source()['site_url'] );
+	}
+
+	public function test_it_rejects_invalid_json(): void {
+		$transport = new JsonFileTransport( new PackageValidator() );
+
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'The selected file is not valid JSON.' );
+
+		$transport->import( '{"schema_version":' );
+	}
+
+	public function test_it_rejects_schema_errors(): void {
+		$transport = new JsonFileTransport( new PackageValidator() );
+
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'records is required.' );
+
+		$transport->import( '{"schema_version":"1.0"}' );
+	}
+
+	private function package(): ContentPackage {
+		$records = array(
+			'posts'             => array(),
+			'terms'             => array(),
+			'media'             => array(),
+			'custom_post_types' => array(),
+		);
+
+		return ContentPackage::fromArray(
+			array(
+				'schema_version' => '1.0',
+				'generated_at'   => '2026-04-26T20:30:00+00:00',
+				'source'         => array(
+					'site_url' => 'https://example.test',
+					'name'     => 'Example Production',
+				),
+				'destination'    => array(
+					'site_url' => 'https://staging.example.test',
+					'name'     => 'Example Staging',
+				),
+				'manifest'       => array(
+					'posts'             => 0,
+					'terms'             => 0,
+					'media'             => 0,
+					'custom_post_types' => 0,
+				),
+				'records'        => $records,
+				'checksums'      => array(
+					'records' => PackageChecksum::records( $records ),
+				),
+			)
+		);
+	}
+}

tests/bootstrap.php

@@ -143,11 +143,12 @@ if ( ! function_exists( 'wp_json_encode' ) ) {
 	 * Minimal JSON encoder for unit tests.
 	 *
 	 * @param mixed $value Value to encode.
+	 * @param int   $flags JSON encoding flags.
 	 * @return string|false
 	 */
-	function wp_json_encode( $value ) {
+	function wp_json_encode( $value, $flags = 0 ) {
 		// phpcs:ignore -- Test stub for WordPress' wp_json_encode().
-		return json_encode( $value );
+		return json_encode( $value, $flags );
 	}
 }