Use platform nodes + route membership for bus stop clustering

Replaces distance-based clustering with route-membership grouping:
- download_osm.py: collect route_refs per stop node; use platform roles
  for bus stops, stop roles for rail stops
- prepare.py: carry route_refs through to master.gpkg
- render.py: group_by_route_membership() uses union-find to cluster
  same-name stops sharing at least one route into one marker; stops on
  disjoint routes are kept separate. No distance threshold needed.

Results: 786 bus platforms -> 465 markers (was 1610 -> 781).
Veksøvej: 2 platforms sharing routes 2A+5C -> 1 marker.
Peter Bangs Vej: 7 platforms on routes 10/21/22/4A/7A -> 2 markers.

copenhagen/scripts/render.py

@@ -262,11 +262,70 @@ def rail_station_names(stations):
     return names
 
 
+def group_by_route_membership(rows, cluster_m):
+    """Group stop rows by (name, shared route_ref) into distinct stops.
+
+    Two same-name stops that share at least one route_ref are the same stop.
+    Stops on disjoint routes are different stops. Within each group,
+    platform duplicates are merged by distance clustering.
+    Returns a list of (x, y) centroids.
+    """
+    if rows is None or rows.empty:
+        return []
+
+    coords = [(r.geometry.x, r.geometry.y) for _, r in rows.iterrows()]
+    route_sets = []
+    for _, r in rows.iterrows():
+        rr = r.get("route_refs")
+        if isinstance(rr, str) and rr:
+            route_sets.append(set(rr.split(";")))
+        else:
+            route_sets.append(set())
+
+    n = len(coords)
+    # union-find: stops sharing a route are connected
+    parent = list(range(n))
+
+    def find(x):
+        while parent[x] != x:
+            parent[x] = parent[parent[x]]
+            x = parent[x]
+        return x
+
+    def union(a, b):
+        ra, rb = find(a), find(b)
+        if ra != rb:
+            parent[ra] = rb
+
+    for i in range(n):
+        for j in range(i + 1, n):
+            if route_sets[i] & route_sets[j]:
+                union(i, j)
+
+    # group indices by connected component
+    components = {}
+    for i in range(n):
+        root = find(i)
+        components.setdefault(root, []).append(i)
+
+    centroids = []
+    for indices in components.values():
+        comp_coords = [coords[i] for i in indices]
+        if len(comp_coords) == 1:
+            centroids.append(comp_coords[0])
+        else:
+            # stops sharing a route are the same stop; merge to centroid
+            import numpy as np
+            centroids.append(tuple(np.mean(comp_coords, axis=0)))
+    return centroids
+
+
 def plot_bus_stops(ax, stops, styling, active_styles, rail_names=None):
     """Draw small markers for bus stops from the stops layer.
 
     Bus stops whose name matches a rail station are skipped — the rail
-    station marker represents that stop.
+    station marker represents that stop. Stops are grouped by shared
+    route membership: same-name stops on disjoint routes are distinct.
     """
     if stops is None or stops.empty or "bus" not in active_styles:
         return
@@ -292,19 +351,14 @@ def plot_bus_stops(ax, stops, styling, active_styles, rail_names=None):
     marker = "o" if shape == "circle" else "s"
     top_z = max(styling["zorder"].values()) + 1
 
-    # cluster same-name bus stops within threshold (opposite sides of road)
-    # skip names that match a rail station (merged into station marker)
+    # group by name, then by route membership; skip rail station names
     centroids = []
     skipped = 0
     for name, grp in bus.groupby("name"):
         if name and name in rail_names:
             skipped += len(grp)
             continue
-        coords = [(r.geometry.x, r.geometry.y) for _, r in grp.iterrows()]
-        if name and len(coords) > 1:
-            centroids.extend(cluster_stops(coords, cluster_m))
-        else:
-            centroids.extend(coords)
+        centroids.extend(group_by_route_membership(grp, cluster_m))
 
     ax.scatter([p[0] for p in centroids], [p[1] for p in centroids],
                s=sz ** 2, marker=marker, c=fill, edgecolors=edge,