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.

tests/test_category_overton.py

@@ -0,0 +1,116 @@
+"""Validate category decomposition data for Overton report."""
+from __future__ import annotations
+
+import duckdb
+import pytest
+
+DB_PATH = "data/motions.db"
+
+
+@pytest.fixture(scope="module")
+def con():
+    c = duckdb.connect(DB_PATH)
+    yield c
+    c.close()
+
+
+def test_category_distribution(con):
+    """There are exactly 10 categories and all 3,030 motions are classified."""
+    df = con.execute("""
+        SELECT category, COUNT(*) as cnt
+        FROM right_wing_motions
+        WHERE classified = TRUE
+        GROUP BY category
+        ORDER BY cnt DESC
+    """).fetchdf()
+    assert len(df) == 10
+    assert df["cnt"].sum() == 3030
+    assert df[df["category"] == "overig"]["cnt"].values[0] >= 100
+
+
+def test_category_deltas(con):
+    """Pre/post CS deltas (2024 split) — ALL categories gained, energie/klimaat leads."""
+    df = con.execute("""
+        SELECT
+          category,
+          AVG(CASE WHEN year < 2024 THEN centrist_support_strict END) as pre_cs,
+          AVG(CASE WHEN year >= 2024 THEN centrist_support_strict END) as post_cs,
+          COUNT(*) as n
+        FROM right_wing_motions
+        WHERE classified = TRUE AND year >= 2017
+        GROUP BY category
+    """).fetchdf()
+
+    assert len(df) == 10
+    df = df.copy()
+    df["delta"] = df["post_cs"] - df["pre_cs"]
+    top = df.sort_values("delta", ascending=False)
+    assert top.iloc[0]["category"] == "energie/klimaat"
+    assert 0.35 < abs(top.iloc[0]["delta"]) < 0.45
+
+    bottom = df.sort_values("delta", ascending=True)
+    assert bottom.iloc[0]["category"] in ("veiligheid/justitie", "onderwijs/wetenschap")
+
+    assert all(df["delta"] > 0)
+
+
+def test_yearly_category_cs(con):
+    """Yearly CS per category returns data for every year-category combo."""
+    df = con.execute("""
+        SELECT year, category, AVG(centrist_support_strict) as cs, COUNT(*) as n
+        FROM right_wing_motions
+        WHERE classified = TRUE AND year >= 2017
+        GROUP BY year, category
+        ORDER BY year, category
+    """).fetchdf()
+    assert len(df) >= 50
+    assert df["category"].nunique() == 10
+    assert df["year"].nunique() >= 8
+
+
+def test_quarterly_category_data(con):
+    """Quarterly CS per key categories returns expected shape."""
+    key_cats = ["asiel/vreemdelingen", "energie/klimaat", "buitenland/europa",
+                "landbouw/natuur", "economie"]
+    df = con.execute("""
+        SELECT
+          EXTRACT(YEAR FROM m.date) AS y,
+          CEIL(EXTRACT(MONTH FROM m.date) / 3.0) AS q,
+          r.category,
+          AVG(r.centrist_support_strict) AS cs,
+          COUNT(*) AS n
+        FROM right_wing_motions r
+        JOIN motions m ON r.motion_id = m.id
+        WHERE r.classified = TRUE AND m.date IS NOT NULL
+          AND r.category IN ({})
+        GROUP BY y, q, r.category
+        ORDER BY y, q
+    """.format(",".join(f"'{c}'" for c in key_cats))).fetchdf()
+
+    assert df["category"].nunique() <= 5
+    for cat in key_cats:
+        assert cat in df["category"].values
+
+
+def test_qmd_has_domain_decomposition_section():
+    """QMD should have a Domain Decomposition heading after implementation."""
+    qmd = open("reports/overton_window/overton_window.qmd").read()
+    assert "## Domain Decomposition" in qmd
+
+
+def test_qmd_has_category_delta_chart():
+    """QMD should have a category delta bar chart cell."""
+    qmd = open("reports/overton_window/overton_window.qmd").read()
+    assert "chart-7-category-delta" in qmd
+
+
+def test_qmd_has_category_filter_dropdown():
+    """Chart 1 should have updatemenu for category filtering."""
+    qmd = open("reports/overton_window/overton_window.qmd").read()
+    assert "updatemenus" in qmd
+
+
+def test_qmd_has_domain_trajectories():
+    """Quarterly chart should have category trajectories."""
+    qmd = open("reports/overton_window/overton_window.qmd").read()
+    assert "chart-8-domain-trajectories" in qmd