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/editor-save.test.js

@@ -1,4 +1,4 @@
-import { describe, it, afterEach, expect, $, click, type, sleep, closeAll, csrf } from './runner.js?v=3';
+import { describe, it, afterEach, expect, $, click, type, sleep, closeAll, csrf } from './runner.js?v=4';
 
 describe('Editor: Empty Title Prevention', () => {
     afterEach(() => closeAll());