fix(analysis): add Procrustes alignment and normalize vectors for drift computation

SVD sign/rotation is arbitrary per window. Without alignment, drift was
dominated by basis flips (~1.9/step max=2.0) rather than real political movement.

- _procrustes_align_windows(): aligns each window to the previous using
  orthogonal Procrustes on common entities (scipy, falls back gracefully)
- compute_trajectories(): builds aligned window dict before per-MP drift calc,
  adds normalize=True (L2-normalise) to remove cross-window magnitude differences
  caused by varying numbers of motions per quarter
- Results now in sensible range: NSC=2.28, DENK=1.90, ... PVV=0.82, FVD=0.70
- NSC large late jump (1.39 in Q4→Q1 2026) matches its parliamentary fracture
- Add outputs/trajectories_party_aligned.html with cleaned-up drift chart

analysis/trajectory.py

@@ -15,9 +15,77 @@ from typing import Dict, List, Optional
 import numpy as np
 import duckdb
 
+try:
+    from scipy.linalg import orthogonal_procrustes as _scipy_procrustes
+
+    _HAS_SCIPY = True
+except ImportError:
+    _scipy_procrustes = None  # type: ignore[assignment]
+    _HAS_SCIPY = False
+
 _logger = logging.getLogger(__name__)
 
 
+def _procrustes_align_windows(
+    window_vecs: Dict[str, Dict[str, np.ndarray]],
+    min_overlap: int = 5,
+) -> Dict[str, Dict[str, np.ndarray]]:
+    """Align SVD vectors across windows using Procrustes rotations.
+
+    Takes the first window as reference and aligns each subsequent window
+    to it via orthogonal Procrustes on the set of common entities.
+
+    Args:
+        window_vecs: {window_id: {entity_id: vector}}
+        min_overlap: minimum number of common entities needed for alignment
+
+    Returns same structure with rotated vectors for windows 1..N.
+    """
+    if not _HAS_SCIPY:
+        _logger.debug("scipy not available, skipping Procrustes alignment")
+        return window_vecs
+
+    window_ids = list(window_vecs.keys())
+    if len(window_ids) < 2:
+        return window_vecs
+
+    result = {window_ids[0]: window_vecs[window_ids[0]]}
+
+    # Accumulate the aligned reference — each window aligns to the *previous aligned* window
+    prev_aligned = window_vecs[window_ids[0]]
+
+    for wid in window_ids[1:]:
+        cur = window_vecs[wid]
+        common = [e for e in cur if e in prev_aligned]
+
+        if len(common) < min_overlap:
+            _logger.debug(
+                "Procrustes skipped for %s: only %d common entities (need %d)",
+                wid,
+                len(common),
+                min_overlap,
+            )
+            result[wid] = cur
+            prev_aligned = cur
+            continue
+
+        ref_mat = np.vstack([prev_aligned[e] for e in common])
+        cur_mat = np.vstack([cur[e] for e in common])
+
+        try:
+            assert _scipy_procrustes is not None
+            R, _ = _scipy_procrustes(cur_mat, ref_mat)
+            aligned = {e: v.dot(R) for e, v in cur.items()}
+        except Exception:
+            _logger.exception("Procrustes failed for window %s", wid)
+            aligned = cur
+
+        result[wid] = aligned
+        prev_aligned = aligned
+
+    return result
+
+
 def _load_window_ids(db_path: str) -> List[str]:
     """Return all distinct window IDs from svd_vectors, in lexicographic order."""
     conn = duckdb.connect(db_path)
@@ -49,15 +117,23 @@ def _load_mp_vectors_for_window(db_path: str, window_id: str) -> Dict[str, np.nd
 def compute_trajectories(
     db_path: str,
     window_ids: Optional[List[str]] = None,
+    normalize: bool = True,
 ) -> Dict[str, Dict]:
     """Compute per-MP trajectories across windows.
 
+    Args:
+        db_path:    Path to DuckDB database.
+        window_ids: Subset of window IDs to use (default: all, ordered).
+        normalize:  If True (default), L2-normalise each vector before computing
+                    drift so that cross-window magnitude differences (caused by
+                    different numbers of motions per window) don't inflate drift.
+
     Returns:
         {
           mp_name: {
             "windows": [window_id, ...],
-            "vectors": [[...], ...],   # one vector per window
-            "drift": [float, ...],     # consecutive Euclidean distances
+            "vectors": [[...], ...],   # one vector per window (raw, not normalised)
+            "drift": [float, ...],     # consecutive Euclidean distances on unit sphere
             "total_drift": float,
           }
         }
@@ -70,12 +146,18 @@ def compute_trajectories(
         _logger.info("Fewer than 2 windows — no trajectories to compute")
         return {}
 
-    # Collect per-window vectors for each MP
-    mp_data: Dict[str, Dict] = {}
-
+    # Collect per-window vectors keyed as {window_id: {entity_id: vector}}
+    raw_window_vecs: Dict[str, Dict[str, np.ndarray]] = {}
     for wid in window_ids:
-        vecs = _load_mp_vectors_for_window(db_path, wid)
-        for mp_name, vec in vecs.items():
+        raw_window_vecs[wid] = _load_mp_vectors_for_window(db_path, wid)
+
+    # Align windows via Procrustes to remove arbitrary SVD sign/rotation flips
+    aligned_window_vecs = _procrustes_align_windows(raw_window_vecs)
+
+    # Reshape into per-MP view
+    mp_data: Dict[str, Dict] = {}
+    for wid in window_ids:
+        for mp_name, vec in aligned_window_vecs[wid].items():
             if mp_name not in mp_data:
                 mp_data[mp_name] = {"windows": [], "vectors": []}
             mp_data[mp_name]["windows"].append(wid)
@@ -87,8 +169,16 @@ def compute_trajectories(
         if len(data["windows"]) < 2:
             continue
         vecs = data["vectors"]
+        if normalize:
+            normed = []
+            for v in vecs:
+                n = np.linalg.norm(v)
+                normed.append(v / n if n > 1e-10 else v)
+        else:
+            normed = vecs
         drifts = [
-            float(np.linalg.norm(vecs[i + 1] - vecs[i])) for i in range(len(vecs) - 1)
+            float(np.linalg.norm(normed[i + 1] - normed[i]))
+            for i in range(len(normed) - 1)
         ]
         result[mp_name] = {
             "windows": data["windows"],