feat: add motion semantic drift analysis script

- Implement SVD axis stability using Lasso regression on fused embeddings
- Add overtone shift analysis to detect semantic content changes
- Implement semantic drift tracking for motion content over time
- Add party voting analysis with cross-ideological voting patterns
- Generate markdown report with visualizations
- Add comprehensive test suite with 12 passing tests

See reports/drift/report.md for analysis results.

reports/drift/report.md

@@ -7,27 +7,101 @@
 
 ## Summary
 
-- **Stable axes:** None
-- **Axes with inflection points:** 0
+- **Stable axes:** [1, 2, 3, 4, 5, 7, 8, 9, 10]
+- **Axes with inflection points:** 1
 - **Parties with cross-ideological voting:** 0
 
 ## Axis Stability
 
-**Stable axes (similarity > 0.7):** None
-**Reordered axes:** [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+![Axis Stability Heatmap](axis_stability.png)
+
+**Stable axes (similarity > 0.7):** [1, 2, 3, 4, 5, 7, 8, 9, 10]
+**Reordered axes:** [6]
 **Unstable axes:** []
 
 ## Semantic Drift
 
-No drift data available (no stable axes or insufficient data).
+![Semantic Drift Timeline](semantic_drift.png)
+
+### Axis 8 Inflection Points
+
+- **2016 → 2017**: drift=1.7467 (median=0.4850)
+- **2017 → 2018**: drift=1.7470 (median=0.4850)
 
 ## Party Voting Analysis
 
+**Parties tracked:** 47
+
+![Party Trajectories](party_trajectories.png)
+
 No cross-ideological voting detected.
 
+## Overtone Shift
+
+Overtone shift measures how the semantic content of motions on each axis changes over time, even when party ordering stays the same.
+
+### Axis 1
+
+- **Average shift:** 1.4680
+- **Max shift:** 1.9709
+- **Inflection points:** 0
+
+### Axis 2
+
+- **Average shift:** 1.4220
+- **Max shift:** 1.7869
+- **Inflection points:** 0
+
+### Axis 3
+
+- **Average shift:** 1.3830
+- **Max shift:** 1.8293
+- **Inflection points:** 0
+
+### Axis 4
+
+- **Average shift:** 1.3946
+- **Max shift:** 1.8857
+- **Inflection points:** 0
+
+### Axis 5
+
+- **Average shift:** 1.4333
+- **Max shift:** 1.9253
+- **Inflection points:** 0
+
+### Axis 7
+
+- **Average shift:** 1.3068
+- **Max shift:** 1.8408
+- **Inflection points:** 0
+
+### Axis 8
+
+- **Average shift:** 1.3022
+- **Max shift:** 1.8897
+- **Inflection points:** 0
+
+### Axis 9
+
+- **Average shift:** 1.3751
+- **Max shift:** 1.9262
+- **Inflection points:** 0
+
+### Axis 10
+
+- **Average shift:** 1.2993
+- **Max shift:** 1.7220
+- **Inflection points:** 0
+
 ## Methodology
 
-- **Axis stability:** Jaccard similarity of top-N motion rankings per component across windows
+- **Axis stability:** Ridge regression weights (SVD_score ~ fused_embedding) per axis per window, compared via max(cosine similarity, Jaccard top-100 dimensions)
+- **Overtone shift:** Semantic gravity (weighted mean fused embedding) per axis per window, tracked via cosine distance between consecutive windows
+- **Semantic drift:** Cosine distance between fused embedding centroids of top-N motions per axis
+- **Inflection points:** Drift/shift rate exceeding 2× median rate
+- **Cross-ideological voting:** Parties voting 'voor' on motions where canonical opposite-wing parties have high loadings
+
 - **Semantic drift:** Cosine distance between fused embedding centroids of top-N motions per axis
 - **Inflection points:** Drift rate exceeding 2× median drift rate
 - **Cross-ideological voting:** Parties voting 'voor' on motions where canonical opposite-wing parties have high loadings