A ScrollFrame built its `<ScrollChild>` after its `<Frames>` children and so after their OnLoads: under a scroll frame template, the template's scroll bar loaded before the instance's scroll child, and an OnLoad that read the other frame saw it unbuilt.
In the reference, the ScrollFrame's load runs the base load, then builds the scroll child with its whole lifecycle and its own OnLoad, then stores it as the scroll child, all before the post-load hook builds `<Frames>` and fires the ScrollFrame's OnLoad. A template's scroll child is built at the template's level, ahead of the instance's, which wins.
The loader now builds `<ScrollChild>` in that place, on the one path XML and `CreateFrame` share, and only for a ScrollFrame.
Tests cover the order directly, through a template, and with a scroll child in both template and instance.
Fixes#846
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
