Add cross-platform clipboard support

Implement CopyFile for darwin, linux, and windows and call it from
main to copy generated output to the clipboard. Enable CGO for builds,
update goreleaser and release workflow to use macOS, and add a
cross-platform test workflow that verifies the clipboard.

clipboard/windows.go

@@ -0,0 +1,103 @@
+//go:build windows
+
+package clipboard
+
+import (
+	"fmt"
+	"path/filepath"
+	"syscall"
+	"time"
+	"unsafe"
+)
+
+var (
+	user32           = syscall.NewLazyDLL("user32.dll")
+	kernel32         = syscall.NewLazyDLL("kernel32.dll")
+	openClipboard    = user32.NewProc("OpenClipboard")
+	closeClipboard   = user32.NewProc("CloseClipboard")
+	emptyClipboard   = user32.NewProc("EmptyClipboard")
+	setClipboardData = user32.NewProc("SetClipboardData")
+	globalAlloc      = kernel32.NewProc("GlobalAlloc")
+	globalLock       = kernel32.NewProc("GlobalLock")
+	globalUnlock     = kernel32.NewProc("GlobalUnlock")
+	globalFree       = kernel32.NewProc("GlobalFree")
+)
+
+const (
+	cfHDrop = 15
+	gHnd    = 0x0042
+)
+
+type dropFiles struct {
+	pFiles uint32
+	pt     struct{ x, y int32 }
+	fNC    int32
+	fWide  int32
+}
+
+func CopyFile(path string) error {
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+
+	pathUTF16, err := syscall.UTF16FromString(absPath)
+	if err != nil {
+		return err
+	}
+	pathUTF16 = append(pathUTF16, 0)
+
+	dropSize := uint32(unsafe.Sizeof(dropFiles{}))
+	dataSize := uint32(len(pathUTF16) * 2)
+	totalSize := uintptr(dropSize + dataSize)
+
+	hMem, _, err := globalAlloc.Call(gHnd, totalSize)
+	if hMem == 0 {
+		return fmt.Errorf("GlobalAlloc failed: %v", err)
+	}
+
+	ptrVal, _, _ := globalLock.Call(hMem)
+	if ptrVal == 0 {
+		_, _, _ = globalFree.Call(hMem)
+		return fmt.Errorf("GlobalLock failed")
+	}
+
+	df := (*dropFiles)(unsafe.Pointer(ptrVal))
+	df.pFiles = dropSize
+	df.fWide = 1
+
+	targetPtr := unsafe.Pointer(ptrVal + uintptr(dropSize))
+
+	srcSlice := unsafe.Slice((*uint16)(unsafe.Pointer(&pathUTF16[0])), len(pathUTF16))
+	dstSlice := unsafe.Slice((*uint16)(targetPtr), len(pathUTF16))
+	copy(dstSlice, srcSlice)
+
+	_, _, _ = globalUnlock.Call(hMem)
+
+	var openSuccess bool
+	for range 10 {
+		ret, _, _ := openClipboard.Call(0)
+		if ret != 0 {
+			openSuccess = true
+			break
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+
+	if !openSuccess {
+		_, _, _ = globalFree.Call(hMem)
+		return fmt.Errorf("clipboard locked")
+	}
+	defer func() {
+		_, _, _ = closeClipboard.Call()
+	}()
+
+	_, _, _ = emptyClipboard.Call()
+
+	if ret, _, err := setClipboardData.Call(uintptr(cfHDrop), hMem); ret == 0 {
+		_, _, _ = globalFree.Call(hMem)
+		return fmt.Errorf("SetClipboardData failed: %v", err)
+	}
+
+	return nil
+}