Restore Avalonia NativeAOT canvas clear compatibility #618

Parent: #227.

Current WebScene main 15138f2eb63e6bd9e912febc9fd7f89cab00c45e fails both macOS ARM64 NativeAOT Avalonia 11/12 lanes at NativeCanvasSceneRenderer.cs:1580: CS1501: No overload for method Clear takes 2 arguments.

Evidence: https://github.com/SceneTech/WebScene/actions/runs/35428781215

Proposed fix

Replace or conditionally adapt the two-argument clear call to the SkiaSharp surface shared by both supported Avalonia profiles. Add a compile/source contract covering both package matrices, then rerun the two focused NativeAOT lanes.