download_media now shells out to gallery-dl (handles X.com auth/403/video->mp4) instead of urllib, returns the downloaded files, + media_files_for_post glob helper + --cookies CLI. render_markdown embeds images with  and videos with an HTML <video controls> tag (were plain links). extract_corpus uses the package download_media (one download path). Tests rewritten for the gallery-dl API. Corpus: 4 threads, 33 media (32 img embeds + 1 video), 0 missing.
Added standalone Result[T] (frozen+slots, ok/err classmethods, is_ok) to error_types.py per canonical AND-over-OR error_handling.md (not the video_analysis _Ok|_Err sum type). render_markdown renders YAML front-matter from root post, per-post ## sections with reply markers, quote blockquotes, and ./media/<name> links; media_names mapping keeps render decoupled from download_media. TDD: red (no Result) -> green (7 render + 9 types passed).
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