Consolidate path safety, temp downloads, and cleanup validation helpers

internal/pathsafe/relative_destination.go

@@ -0,0 +1,38 @@
+package pathsafe
+
+import (
+	"errors"
+	"path"
+	"regexp"
+	"strings"
+)
+
+var (
+	ErrRelativePathRequired = errors.New("relative path is required")
+	ErrRelativePathAbsolute = errors.New("relative path must not be absolute")
+	ErrRelativePathEscape   = errors.New("relative path escapes root")
+)
+
+var windowsAbsPathRE = regexp.MustCompile(`^[a-zA-Z]:[\\/]`)
+
+// NormalizeRelativeDestination validates and normalizes a relative destination
+// path to slash-separated form.
+func NormalizeRelativeDestination(relative string) (string, error) {
+	trimmed := strings.TrimSpace(relative)
+	if trimmed == "" {
+		return "", ErrRelativePathRequired
+	}
+	if strings.HasPrefix(trimmed, "/") || strings.HasPrefix(trimmed, `\`) || windowsAbsPathRE.MatchString(trimmed) {
+		return "", ErrRelativePathAbsolute
+	}
+
+	normalized := strings.ReplaceAll(trimmed, `\`, "/")
+	cleaned := path.Clean(normalized)
+	if cleaned == "." || cleaned == "" {
+		return "", ErrRelativePathRequired
+	}
+	if cleaned == ".." || strings.HasPrefix(cleaned, "../") {
+		return "", ErrRelativePathEscape
+	}
+	return cleaned, nil
+}