runner: bump RUNNER_V ?v=3 → ?v=4 so test-shield removal loads

The test-shield (#test-shield) was added in loader.js (always fresh
via plugin manager ?v=time()) but the removal lives in runner.js
hideProgress(). runner.js was still cached at ?v=3 in the browser,
so the shield stayed after the run. Bumping to ?v=4 in loader.js +
all 42 test files forces a fresh runner.js load.

chore: bump version to 0.9.15

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/tests/loader.js

@@ -39,7 +39,7 @@ document.addEventListener('keydown', e => {
 
 // Must match the `?v=…` suffix used in test files' `from './runner.js?v=…'`
 // imports — bump both in lock-step to invalidate the runner.js cache.
-const RUNNER_V = '?v=3';
+const RUNNER_V = '?v=4';
 
 function ensureProgressBar() {
     if (document.getElementById('test-progress')) return;