Internal deterministic downloader used by AwesomeWebpageMetaSkill after the
bounded web-search step has produced candidate image URLs.
It receives normalized media slots and search output on stdin, downloads direct
image URLs with Python HTTP APIs, validates the response MIME/magic bytes, saves
files by slot_id, and emits IMAGE_READY: records plus
IMAGE_DOWNLOAD_INCOMPLETE: when any requested image slot remains unfilled.