feat(dx12): DirectComposition path for per-pixel alpha (#298)

[kolkov]

Summary

• Add DirectComposition (DComp) path for DX12 swap chain creation, enabling per-pixel alpha transparency (CompositeAlphaModePremultiplied)
• CreateSwapChainForHwnd only supports DXGI_ALPHA_MODE_IGNORE — per-pixel alpha requires CreateSwapChainForComposition via DirectComposition visual tree
• Lazy-load dcomp.dll via sync.Once (Windows 8+, always present on our min Win10 1809)
• Auto-select: AlphaMode == Premultiplied → DComp path, Opaque → HWND path (unchanged)
• Fix adapter to report Premultiplied only when dcompAvailable()
• GOGPU_DX12_FORCE_HWND=1 env var for RenderDoc compatibility

Changes (5 files, +507/-27 LOC)

File	Change
hal/dx12/dcomp.go	New. DComp COM bindings: lazy dll loading, IDCompositionDevice/Target/Visual vtbl, dcompState lifecycle (init/bindSwapChain/release)
hal/dx12/dxgi/factory.go	CreateSwapChainForComposition wrapper on IDXGIFactory6
hal/dx12/surface.go	createSwapchain branching: DComp vs HWND based on AlphaMode. MakeWindowAssociation guarded for DComp path (Rust wgpu parity)
hal/dx12/instance.go	dcomp *dcompState field on Surface. Teardown in Unconfigure. Alpha mode change detection in Configure (forces full recreate instead of ResizeBuffers)
hal/dx12/adapter.go	Both Adapter and AdapterLegacy: Premultiplied conditional on dcompAvailable()

Architecture (ADR-057)

AlphaMode == Premultiplied:
  DCompositionCreateDevice2(NULL) → device
    → CreateTargetForHwnd(hwnd, false) → target
    → CreateVisual() → visual → target.SetRoot(visual)
  factory.CreateSwapChainForComposition(queue, desc) → swapchain
  visual.SetContent(swapchain) → device.Commit()

AlphaMode == Opaque:
  factory.CreateSwapChainForHwnd(queue, hwnd, desc) → swapchain  (unchanged)

Enterprise review findings (all addressed)

• vtbl index 15 for SetContent: verified against Windows SDK + Rust wgpu
• IID GUID {C37EA93A-E7AA-450D-B16F-9746CB0407F3}: verified
• All error paths clean up DComp + swapchain resources: verified (5 paths)
• Teardown order: swapchain → visual → target → device: verified
• Alpha mode change during resize: detected, forces full recreate
• MakeWindowAssociation: skipped on DComp path (Rust wgpu parity)
• ResizeBuffers preserves COM identity, DComp visual retains reference: verified

Requires (separate repo)

gogpu#361: WS_EX_NOREDIRECTIONBITMAP window style + AlphaMode passthrough in renderer. Without this, DComp swap chain works but transparency is not visible (DWM redirection surface provides opaque background).

Test plan

• go build ./... (Windows, Linux, macOS)
• golangci-lint run --timeout=5m — 0 issues
• gofmt — clean
• Cross-platform build: GOOS=linux, GOOS=darwin
• Visual: Opaque window renders correctly (regression)
• Visual: Premultiplied + WS_EX_NOREDIRECTIONBITMAP → transparent (requires gogpu#361)
• Visual: GOGPU_DX12_FORCE_HWND=1 falls back to HWND path

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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