Add retained GPU canvases and publish the Canvas Lab

[ExtraBinoss]

Summary

• add retained GPU canvas registration, device/render contexts, caching, diagnostics and runtime integration
• add the native and WebAssembly GPU Canvas Lab with natural/inverted vertical dragging and smooth zoom
• publish the responsive Canvas Lab on the website App Examples page
• document the API, limitations and user-facing changes in the changelog

Verification

• ./scripts/linux-hidden-display.sh env ARGUI_NATIVE_TESTS=1 ./scripts/quality.sh
• 1,324 tests passed; 2 skipped
• workspace coverage: 89.24% branches, 95.84% lines
• release Wasm build and static website preview verified on desktop and mobile

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