feat(wgpu): select backends via InstanceExtras, add SwapChainPanel surface

[YindSoft]

Summary

Two small additions against the wgpu-native v29 header:

• InstanceDescriptor.NativeBackends — when non-zero, CreateInstance chains WGPUInstanceExtras (full v29 layout, 112 bytes) with backends set. Until now the descriptor was ignored, so there was no way to choose a backend (wgpu-native does not read WGPU_BACKEND). Zero keeps today's behavior.
• Instance.CreateSurfaceFromSwapChainPanel (Windows) — wraps WGPUSurfaceSourceSwapChainPanel. wgpu-native creates the swap chain with CreateSwapChainForComposition and hands it to ISwapChainPanelNative::SetSwapChain. Besides a WinUI SwapChainPanel, any object implementing that interface works, so the caller can put the swap chain in its own DirectComposition tree.

Motivation

We render a game with this binding under a WebView2 composition controller (DirectComposition, topmost target on the same HWND). With an HWND swap chain, going fullscreen let DWM / the driver promote the swap chain to independent flip and the WebView2 layer stopped being composed, so the UI disappeared. On NVIDIA the default adapter was Vulkan, which does the same. Forcing DX12 (NativeBackends: InstanceBackendDX12) and presenting a composition swap chain through a small Go ISwapChainPanelNative whose SetSwapChain calls IDCompositionVisual::SetContent keeps the game and the UI composed together in fullscreen.

Notes

• Both chained structs are heap-allocated through package-level sinks, because the descriptors only refer to them as uintptr and a stack copy could move before the native call reads it.
• Tests: size/offset tests for instanceExtrasWire, nativeDisplayHandleWire and surfaceSourceSwapChainPanel, the SurfaceSourceSwapChainPanel SType value, TestCreateInstanceWithNativeBackends, and a nil-panel rejection test.
• On main, go test ./wgpu with the v29 DLL aborts at TestCreateBindGroupLayout, before and after this change. That is the bind group layout ABI bug fixed in fix(wgpu): v29 wire layouts, surface texture on error, pinned pointers in Proc.Call #32. With this commit cherry-picked on top of fix(wgpu): v29 wire layouts, surface texture on error, pinned pointers in Proc.Call #32, the full suite passes on Windows (ok github.com/go-webgpu/webgpu/wgpu 27.5s). go vet passes for windows, linux and darwin.
• The CHANGELOG entry goes under a new Unreleased section, so it will need a trivial rebase against fix(wgpu): v29 wire layouts, surface texture on error, pinned pointers in Proc.Call #32.

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