fix: enable Windows cross-compilation

Move syscall.Stat_t usage to platform-specific files since it's
Unix-only. On Windows, getFileOwnership() returns 0, 0 as UID/GID
are Unix concepts.

This allows the SDK to be cross-compiled for Windows targets.

Author: hiroTamada

lib/cp.go

@@ -13,7 +13,6 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
-	"syscall"
 	"time"
 
 	"github.com/gorilla/websocket"
@@ -183,10 +182,7 @@ func cpToInstanceInternal(ctx context.Context, cfg CpConfig, opts CpToInstanceOp
 	// Get UID/GID if archive mode is enabled
 	var uid, gid uint32
 	if opts.Archive {
-		if stat, ok := srcInfo.Sys().(*syscall.Stat_t); ok {
-			uid = stat.Uid
-			gid = stat.Gid
-		}
+		uid, gid = getFileOwnership(srcInfo)
 	}
 
 	// Send initial request

lib/stat_unix.go

@@ -0,0 +1,16 @@
+//go:build unix
+
+package lib
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+// getFileOwnership returns the UID and GID of a file on Unix systems.
+func getFileOwnership(info fs.FileInfo) (uid, gid uint32) {
+	if stat, ok := info.Sys().(*syscall.Stat_t); ok {
+		return stat.Uid, stat.Gid
+	}
+	return 0, 0
+}

lib/stat_windows.go

@@ -0,0 +1,10 @@
+//go:build windows
+
+package lib
+
+import "io/fs"
+
+// getFileOwnership returns 0, 0 on Windows as UID/GID are Unix concepts.
+func getFileOwnership(info fs.FileInfo) (uid, gid uint32) {
+	return 0, 0
+}