Add automated tests for the 25% pricing formula and $25/$15 shipping thresholds

Launch gate (design doc §5/§8) requires automated tests for both; the repo
had zero test infra until now. PHPUnit + Brain Monkey, run via `make test`
against the existing wordpress dev container.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MqhgnCRjjVLFnZ3Jpn93Vg

wp-content/plugins/bookstore-core/composer.json

@@ -5,9 +5,18 @@
     "require": {
         "php": ">=8.1"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^10",
+        "brain/monkey": "^2.6"
+    },
     "autoload": {
         "psr-4": {
             "Bookstore\\Core\\": "includes/"
         }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Bookstore\\Core\\Tests\\": "tests/"
+        }
     }
 }