Add per-row include flag to TSV; expand flies_analysis_simple narrative

- export_video_db_index.py now writes a boolean `include` column
  (default True). Flip it to False to drop a noisy/unusable row from
  analysis without deleting it.
- load_roi_data filters on `include` automatically (back-compat:
  missing column = load everything).
- flies_analysis_simple.ipynb section headers now explain *why* each
  step exists (barrier alignment, body-area baseline, merged-blob
  heuristic, Hungarian identity tracking) rather than just naming
  the step.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/export_video_db_index.py

@@ -165,6 +165,12 @@ def main() -> None:
     df["testing_video_path"] = test_videos
     df["testing_db_path"] = test_dbs
 
+    # Reason: an analyst flag for excluding individual fly/session rows that
+    # turn out to be too noisy or otherwise unusable. Default True; flip to
+    # False directly in the TSV to drop a row from analysis without having
+    # to delete it. load_roi_data() respects this flag.
+    df["include"] = True
+
     df.to_csv(args.out, sep="\t", index=False)
 
     n_rows = len(df)