feat: Windows capture pipeline via GDI BitBlt (purego)

[devin-ai-integration]

Summary

Adds a working Windows pipeline so captured runs on runtime.GOOS == "windows" instead of fataling on unsupported platform.

pipelines/windows/pipeline.go — implements pipelines.Pipeline with plain GDI, bound via purego (syscall.NewLazyDLL + purego.RegisterLibFunc, the same no-cgo pattern the linux backend uses for libdrm):

• ListDisplays: EnumDisplayMonitors + GetMonitorInfoW (callback via purego.NewCallback); IDs are enumeration indexes, X/Y are virtual-desktop coords so negative offsets work.
• StartStream → gdiGrabber: GetDC(0) + BitBlt(SRCCOPY|CAPTUREBLT) into a 32bpp top-down CreateDIBSection bitmap, so pixels are read in place as packed BGRA — no GetDIBits round trip. Alpha byte is forced to 0xFF since GDI leaves it 0. Frames stream on the same ticker cadence model as the linux frameStream.
• SetProcessDPIAware at bind time so enumeration/capture use physical pixels.
• DXGI Desktop Duplication (GPU-side, dirty rects, cursor) is the intended follow-up for performance; GDI is the spike path that gets the socket pipeline working end-to-end.

Platform gating: sckit-go uses purego.Dlopen, which only exists on Unix — so the macos package now sits behind //go:build darwin with a !darwin stub, matching the linux package's existing stub_other.go pattern. The windows impl is tagged windows with a !windows stub.

main.go: case "windows": windows.New(); socket paths go through socketPath(name) → os.TempDir() on Windows (AF_UNIX sockets are filesystem-backed there since Windows 10 17063), /tmp unchanged elsewhere.

Verified

Built and run on Windows Server 2022: info, list-displays (1280x720 display), start-stream at 5fps, and a real 1280x720 BGRA frame read off the media socket with opaque alpha; stop-stream clean. go vet/go build pass on windows, linux, and darwin.

Link to Devin session: https://app.devin.ai/sessions/5e1930eab2f0494cb95bac679adfd552
Open in Devin Desktop: https://app.devin.ai/desktop/session/5e1930eab2f0494cb95bac679adfd552?variant=devin
Requested by: @spacedouut

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