Fix race condition in parallel loadScript/loadCSS calls

**Problem:**
When `loadScript` or `loadCSS` is called in parallel with the same URL, the functions resolve too early because they check for existing script/link tags but don't wait for pending loads.

**Related Issue:** adobe/aem-boilerplate#399

Search the repository for the loadScript and loadCSS entry points and inspect how they handle existing script or link tags. Reproduce parallel calls for the same URL, then ensure completion is delayed until the pending load finishes; add or update a focused test if the repository has coverage for these helpers.

Written by the indexing model from the issue text.