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/load_roi_data.py

@@ -70,6 +70,12 @@ def load_roi_data(meta: pd.DataFrame | None = None) -> pd.DataFrame:
     if meta is None:
         meta = pd.read_csv(VIDEO_INFO_TSV, sep="\t")
 
+    # Honor the per-row `include` flag if the TSV has one. Rows with
+    # include=False are dropped (typically too-noisy videos the analyst
+    # has marked out). Missing column → load everything (back-compat).
+    if "include" in meta.columns:
+        meta = meta[meta["include"].astype(bool)]
+
     db_cache: dict = {}
     chunks: list[pd.DataFrame] = []