feat(overton): add category domain decomposition with interactive charts and TDD tests

Populated the right_wing_motions.category column (previously 100% NULL across
3,030 motions) via parallel subagent classification — 80 agents derived a
10-category taxonomy and classified all motions in minutes.

Adds to the Overton QMD report:
- Plotly dropdown filter on Chart 1 to toggle between policy categories
- Chart 7: category delta bar chart (pre/post centrist support per domain)
- Chart 8: quarterly domain trajectories for the 5 largest categories
- Domain Decomposition narrative section

Also fixes a Streamlit tab crash (m.text -> m.body_text) and adds TDD tests.

analysis/tabs/overton.py

@@ -182,7 +182,7 @@ def _render_motion_browser(con: duckdb.DuckDBPyConnection) -> None:
     st.subheader("Right-Wing Motions Browser")
 
     df = con.execute("""
-        SELECT r.year, r.title, m.text, r.centrist_support_strict, r.category
+        SELECT r.year, r.title, m.body_text, r.centrist_support_strict, r.category
         FROM right_wing_motions r
         LEFT JOIN motions m ON r.motion_id = m.id
         WHERE r.classified = TRUE