Joint UMAP refit with transcripts, density flattening, and welcome screen redesign

- Re-fit joint UMAP on 257K documents (250K articles + 2.5K questions + 5.4K transcripts)
- Apply density flattening (mu=0.85) to all coordinates via title/ID matching
- Rebuild video catalog windows with displacement field interpolation (85K windows)
- Replace welcome screen dropdown with "Map my knowledge!" button
- Fix reset bug: clear video markers when returning to welcome screen
- Update flatten/apply scripts for new coordinate file conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jeremymanning

.gitignore

@@ -229,7 +229,9 @@ embeddings/umap_bounds.pkl
 embeddings/question_embeddings_2500.pkl
 embeddings/transcript_embeddings.pkl
 embeddings/article_coords.pkl
+embeddings/article_coords_flat.pkl
 embeddings/question_coords.pkl
+embeddings/question_coords_flat.pkl
 embeddings/transcript_coords.pkl
 embeddings/article_registry.pkl
 *.credentials
@@ -303,6 +305,7 @@ backups/large_checkpoints/level_1_after_download.json
 
 # Video pipeline working files
 data/videos/.working/coordinates/
+data/videos/.working/embeddings/
 data/videos/.working/audio_cache/
 data/videos/transcripts_raw/
 data/videos/transcripts/