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/phpunit.xml.dist

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.5/phpunit.xsd"
+         bootstrap="tests/bootstrap.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="Unit">
+            <directory>tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>