Cache the surface ring's drop shadow to cut per-frame GPU cost

[danielchalmers]

Summary

The surface ring carries a DropShadowEffect with BlurRadius=32 and RenderingBias="Quality", and it sits underneath every animating element in the menu. WPF re-evaluates the multi-pass Gaussian blur whenever the scene recomposes, so the blur runs during hover/press transitions, drag-to-reorder, and the window fades, which is exactly when the per-frame budget matters.

This wraps the shadowed ring in a container with a BitmapCache (rendered at the current DPI scale so it stays crisp under display scaling), so the blur is rasterized once per menu build and then composited as a texture on every subsequent frame.

Notes

• RenderingBias stays at Quality: with the result cached, the high-quality blur only costs once per rebuild instead of once per frame, so there's no reason to trade it away.
• The high-contrast path (which has no shadow) is unchanged.
• Verified with an offscreen render comparison of the old vs. new code path: the shadow gradient matches within 8/255 per channel, and the only larger deltas are sub-pixel antialiasing on the 1px silhouette edge of the ring where the cached bitmap composites. Nothing off-edge changed.