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-tools.php

@@ -0,0 +1,68 @@
+<?php
+/**
+ * Detects + caches paths to the external image binaries.
+ *
+ * @package H4B\ImageOptim
+ */
+
+namespace H4B\ImageOptim;
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+final class Tools {
+
+	private const REQUIRED = [ 'cwebp', 'cjpeg', 'djpeg', 'jpegtran', 'jpegoptim' ];
+	private const OPTIONAL = [ 'avifenc', 'avifdec', 'pngquant', 'optipng' ];
+
+	/**
+	 * @return array<string,string|null> Map of tool name → absolute path or null.
+	 */
+	public static function detect(): array {
+		static $cache = null;
+		if ( null !== $cache ) {
+			return $cache;
+		}
+		$cache = [];
+		foreach ( array_merge( self::REQUIRED, self::OPTIONAL ) as $bin ) {
+			$cache[ $bin ] = self::which( $bin );
+		}
+		return $cache;
+	}
+
+	public static function path( string $bin ): ?string {
+		$tools = self::detect();
+		return $tools[ $bin ] ?? null;
+	}
+
+	public static function missing_required(): array {
+		$tools = self::detect();
+		$missing = [];
+		foreach ( self::REQUIRED as $bin ) {
+			if ( empty( $tools[ $bin ] ) ) {
+				$missing[] = $bin;
+			}
+		}
+		return $missing;
+	}
+
+	public static function has_avif(): bool {
+		return ! empty( self::path( 'avifenc' ) );
+	}
+
+	private static function which( string $bin ): ?string {
+		// Common locations first, then fall back to PATH lookup.
+		foreach ( [ '/usr/bin/', '/usr/local/bin/', '/opt/local/bin/' ] as $prefix ) {
+			$candidate = $prefix . $bin;
+			if ( is_executable( $candidate ) ) {
+				return $candidate;
+			}
+		}
+		if ( ! function_exists( 'shell_exec' ) ) {
+			return null;
+		}
+		$out = trim( (string) @shell_exec( 'command -v ' . escapeshellarg( $bin ) . ' 2>/dev/null' ) );
+		return $out !== '' && is_executable( $out ) ? $out : null;
+	}
+}