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.
fetch_thread_from_url now passes -o conversations=true (full threads, not single tweets), sorts posts chronologically, and sets root_post_id to the URL's target tweet. render_markdown builds front-matter from the target (root_post_id) post, not the earliest conversation tweet (fixes wrong @handle). Added dedupe_corpus.py: keeps deepest thread per conversation, deletes duplicates, keeps+marks unique branches (branch_of) + writes threads_index.json. 33 tests pass.
thread_from_dict (JSON wire boundary) round-trips thread_data.json; download_media/render_markdown gain argparse main() + __main__ so the plan Task 5.3 pipeline runs via -m; dual-import added so all modules run standalone or via -m (G5). Pipeline integration test (html->names->render) + CLI tests cover the glue without network. TDD: red -> green (4 pipeline + 29 regression = 33 passed).
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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