PatreonDownloader.download_post: writes resolved MediaItems to gallery-dl's
exact on-disk layout (<slug>/patreon/<date>_<id>_<title40>/<NN>_<file>) +
a sidecar the importer's find_sidecar/parse_sidecar consume unchanged. Two-tier
skip (injected seen predicate, then disk). Streamed GET (.part→rename) +
file_validator quarantine; Mux/m3u8 video shells out to yt-dlp with Patreon
Referer/Origin. Pure (no DB) — ledger + orchestration land in step 3. Unit
tests stub the session + yt-dlp seams (no network/subprocess).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
