fix: overlay and content swapped on container elements

[x401om]

Summary

• background decoded from wrong key: Schema.Element.Properties decoded background overlays from .overlay key instead of .background, making the background property completely non-functional
• Overlay/content swap on containers: For all container elements (stack, button, box, row, column, section, pager), the LIFO task stack planning order caused overlay and content results to be swapped during build — overlay got the content, and content got the overlay

Root cause

In planElement, content was planned before planElementProperties (overlay/background). Since tasks are processed LIFO, overlay was built first (result at bottom of result stack), then content (result on top). During buildElement, buildElementProperties popped from the top — getting content instead of overlay — and buildBox/etc. popped next — getting overlay instead of content.

Fix

1. Schema.Element.Properties.swift:146 — changed .overlay → .background for the background decoding key
2. Schema.Element.swift:61-88 — swapped planning order: planElementProperties now comes before planBox/planStack/etc. for all 7 container element types

Test plan

• Verify overlay badges render correctly on leaf elements (image, text) — should be unchanged
• Verify overlay badges render correctly on container elements (box with content + overlay)
• Verify background property works (renders behind the element)
• Test with library/elements/overlay_badges.bundle in the Gallery app

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