feat(wpmc S7): vendor Plugin Update Checker v5.6

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

includes/lib/plugin-update-checker/composer.json

@@ -0,0 +1,23 @@
+{
+	"name": "yahnis-elsts/plugin-update-checker",
+	"type": "library",
+	"description": "A custom update checker for WordPress plugins and themes. Useful if you can't host your plugin in the official WP repository but still want it to support automatic updates.",
+	"keywords": ["wordpress", "plugin updates", "automatic updates", "theme updates"],
+	"homepage": "https://github.com/YahnisElsts/plugin-update-checker/",
+	"license": "MIT",
+	"authors": [
+		{
+			"name": "Yahnis Elsts",
+			"email": "whiteshadow@w-shadow.com",
+			"homepage": "https://w-shadow.com/",
+			"role": "Developer"
+		}
+	],
+	"require": {
+		"php": ">=5.6.20",
+		"ext-json": "*"
+	},
+	"autoload": {
+		"files": ["load-v5p6.php"]
+	}
+}