Rebuild from model

[wieslawsoltes]

Spec

• Add SKSvg.RebuildFromModel() to rebuild the picture from the current model, clear wireframe cache, and return null when Model is missing.
• Make SKSvg.Picture lazily build from Model and cache the result.
• Ensure Avalonia.Svg.Skia.SvgSource retains a SKSvg instance for all load paths, including LoadFromSvg and LoadFromSvgDocument.
• Add SvgSource.RebuildFromModel() in Svg.Controls.Skia.Avalonia to rebuild via SKSvg and refresh Picture.
• Add SvgSource.RebuildFromModel() in Svg.Controls.Avalonia to deep-clone the current picture to refresh the rendered reference.
• Update Avalonia samples with a model-editing demo (grayscale) that calls RebuildFromModel().
• Add unit tests covering rebuild behavior in SKSvg and both SvgSource variants.

API/Implementation

• SKSvg.Picture now lazily materializes _picture from Model; RebuildFromModel() rebuilds the SkiaSharp picture and clears the wireframe cache.
• Avalonia.Svg.Skia.SvgSource now keeps a SKSvg instance for SVG string/document loads and exposes RebuildFromModel() that regenerates the picture from the model.
• Avalonia.Svg.SvgSource exposes RebuildFromModel() that deep-clones the current ShimSkiaSharp.SKPicture to refresh render caches.
• Samples add a "Model" tab showing original vs. modified (grayscale) output by editing draw commands and rebuilding.
• Tests validate rebuild behavior and load retention for SKSvg and both Avalonia SvgSource implementations.

Fixes #450