Graphics: choose swapchain composite alpha and format from surface capabilities #239

Current State

CompositeAlpha is hardcoded to OpaqueBitKhr (SwapChain.Resources.cs) and the format selection prefers B8G8R8A8Srgb, otherwise takes formats[0] (SwapChain.Selection.cs:5-40). Android usually offers R8G8B8A8 and often only INHERIT composite alpha.

Tasks

• Pick composite alpha from supportedCompositeAlpha (Opaque, then Inherit, then any)
• Prefer any 8-bit sRGB format (B8G8R8A8_SRGB, R8G8B8A8_SRGB) before falling back
• Extract both selections into pure functions with unit tests over fake capability sets

Acceptance Criteria

• Unit tests cover Android-like and desktop-like capability sets
• Desktop behavior unchanged

Related

• Android plan task A5b · seniority Jr · mergeable now (.NET 10)
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