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

.gitignore

@@ -7,6 +7,7 @@
 /.poll-deploy-*.lock
 vendor/
 node_modules/
+.phpunit.result.cache
 *.log
 .DS_Store
 /.claude/