add custom stub to demonstration --manifest option capability

examples/app-fixtures/app-fixtures-box.json

@@ -1,5 +1,6 @@
 {
     "main": null,
+    "stub": "app-fixtures-stub.php",
     "output": "app-fixtures.phar",
     "directories": ["bin", "src", "vendor"],
     "directories-bin": ["../../vendor/humbug/box/res/requirement-checker"],

examples/app-fixtures/app-fixtures-stub.php

@@ -0,0 +1,33 @@
+#!/usr/bin/env php
+<?php
+// app-fixtures-stub.php
+/*
+ * Generated by Humbug Box 4.3.8@5534406.
+ *
+ * @link https://github.com/humbug/box
+ */
+
+Phar::mapPhar('app-fixtures-alias.phar');
+
+if ($argc > 1 && $argv[1] === '--manifest') {
+    $resources = $argc > 2 ? [$argv[2]] : ['manifest.txt', 'manifest.xml', 'sbom.xml', 'sbom.json'];
+
+    foreach ($resources as $resource) {
+        $filename = "phar://app-fixtures-alias.phar/{$resource}";
+        if (file_exists($filename)) {
+            echo (file_get_contents($filename));
+            $status = 0;
+            break;
+        } elseif (count($resources) === 1) {
+            echo sprintf('Manifest "%s" is not available in this PHP Archive.', $resource), PHP_EOL;
+            $status = 1;
+            break;
+        }
+    }
+    exit($status);
+}
+
+require 'phar://app-fixtures-alias.phar/.box/bin/check-requirements.php';
+require 'phar://app-fixtures-alias.phar/index.php';
+
+__HALT_COMPILER(); ?>