Implements the retail floating-window bevel as a UiPanel subclass using
RetailChromeSprites: 4 tiled edges + 4 stretched corners + tiled center fill,
matching the 8-piece border layout confirmed by the D.2b Step-0 prove-out.
Resolver delegate keeps GL out of unit tests. Geometry verified by
ComputeFrameRects_PlacesCornersEdgesAndCenter (1/1 pass).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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