Copying a revealed nsec (or a post-rotation new key) left the secret sitting in
the clipboard indefinitely. Add copy_secret_to_buffer: it copies now and, after
a short delay, clears the clipboard only if it still holds exactly that secret
(compare-then-clear), so whatever the user copied since is never clobbered. Both
nsec copy sites now use it. Desktop clears through the long-lived clipboard owner
on a background thread; Android does the same over its JNI clipboard via the
established non-GUI app-handle pattern; other platforms fall back to a plain copy.
2ro
ef4a7a5d19