fix(update): count only consecutive stalls so slow downloads never give up

Refine the resumable download so a slow-but-progressing transfer keeps
resuming for as long as it makes forward progress: the retry budget now
counts only consecutive *stalled* attempts (no new bytes), and any progress
resets it. This guarantees a slow link completes even if it takes far more
than DOWNLOAD_MAX_ATTEMPTS resumes, while a genuinely hung connection is
still bounded. Adds a test that drops the connection repeatedly but always
advances and must still reconstruct the full payload.