feat: initial v0.1.0 MVP

Replaces Smush Pro's optimisation pipeline without the grey-wash bug.

CLI commands working:
  wp h4b-img status
  wp h4b-img optimise --id=<n>
  wp h4b-img bulk
  wp h4b-img rescue

Verified on dev.rds.ink:
- ICC profile preservation works (the Smush-bug fix)
- Bulk: 20 attachments → 487 KB saved (10.4%), 0 errors
- Rescue: end-to-end mechanism verified on WorkingAsOne_horse fixture
- WebP synchronous, AVIF queued via WP-Cron
- Originals backed up to wp-content/h4b-img-originals/

See CHANGELOG.md for details + ../DESIGN-h4b-image-optim.md for architecture.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/class-attachment-meta.php

@@ -0,0 +1,71 @@
+<?php
+/**
+ * Tracks per-attachment optimisation history.
+ *
+ * Stored as a single postmeta key `_h4b_img_optim` containing:
+ *   [
+ *     'version'      => '0.1.0',
+ *     'processed_at' => 'ISO-8601',
+ *     'tool_chain'   => ['imagick','cjpeg','jpegoptim',...],
+ *     'sizes'        => [
+ *        'full'      => ['before'=>123, 'after'=>78, 'webp'=>45, 'avif'=>30, 'percent'=>37],
+ *        '800x524'   => [...],
+ *     ],
+ *     'backup_path'  => 'wp-content/h4b-img-originals/2026/02/...',
+ *     'errors'       => [],
+ *   ]
+ *
+ * @package H4B\ImageOptim
+ */
+
+namespace H4B\ImageOptim;
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+final class Attachment_Meta {
+
+	public const META_KEY = '_h4b_img_optim';
+
+	public static function get( int $attachment_id ): array {
+		$meta = get_post_meta( $attachment_id, self::META_KEY, true );
+		return is_array( $meta ) ? $meta : [];
+	}
+
+	public static function set( int $attachment_id, array $meta ): void {
+		update_post_meta( $attachment_id, self::META_KEY, $meta );
+	}
+
+	public static function is_processed( int $attachment_id ): bool {
+		$meta = self::get( $attachment_id );
+		return ! empty( $meta['processed_at'] );
+	}
+
+	public static function record_size( int $attachment_id, string $size_key, array $stats ): void {
+		$meta              = self::get( $attachment_id );
+		$meta['sizes']     = $meta['sizes'] ?? [];
+		$meta['sizes'][ $size_key ] = $stats;
+		$meta['version']      = H4B_IMG_OPTIM_VERSION;
+		$meta['processed_at'] = current_time( 'c' );
+		self::set( $attachment_id, $meta );
+	}
+
+	public static function mark_avif_pending( int $attachment_id, string $size_key ): void {
+		$meta = self::get( $attachment_id );
+		$meta['avif_pending'] = $meta['avif_pending'] ?? [];
+		$meta['avif_pending'][ $size_key ] = current_time( 'c' );
+		self::set( $attachment_id, $meta );
+	}
+
+	public static function clear_avif_pending( int $attachment_id, string $size_key ): void {
+		$meta = self::get( $attachment_id );
+		if ( isset( $meta['avif_pending'][ $size_key ] ) ) {
+			unset( $meta['avif_pending'][ $size_key ] );
+			if ( empty( $meta['avif_pending'] ) ) {
+				unset( $meta['avif_pending'] );
+			}
+			self::set( $attachment_id, $meta );
+		}
+	}
+}