Mask out Skyttehøj and everything south of it (lat 55.628)

Add south_bound_lat to modes.yaml and a load_south_bound() helper in
prepare.py. When set, the area geometry is intersected with the north
half-plane above the cutoff, so all downstream clipping (lines, stops,
stations) cascades automatically.

Removed: 94 bus stops (including Skyttehøj), 1 station (Vestamager),
and line segments south of the boundary (Metro M1 tail, buses 32-36).

copenhagen/config/modes.yaml

@@ -30,6 +30,11 @@ exclude:
   - agency: Snälltåget AB
     ref: "083"
 
+# South bound: drop every stop/station south of this latitude (inclusive),
+# and clip transit lines at this boundary. Used to cut the map short of
+# Skyttehøj (Amager Landevej) and everything south of it on Amager.
+south_bound_lat: 55.628
+
 modes:
   metro:
     agencies: [Metroselskabet]

copenhagen/scripts/prepare.py

@@ -30,6 +30,7 @@ import sys
 import geopandas as gpd
 import pandas as pd
 import yaml
+from shapely.geometry import box
 
 from _common import CONFIG, GTFS_RAW, PROCESSED
 
@@ -69,6 +70,15 @@ def load_bus_whitelist():
     }
 
 
+def load_south_bound():
+    """South latitude cutoff from modes.yaml, or None.
+
+    When set, stops/stations south of (and transit lines below) this
+    latitude are dropped. Used to trim the map at a boundary.
+    """
+    return _load_modes_yaml().get("south_bound_lat")
+
+
 def _load_modes_yaml():
     return yaml.safe_load((CONFIG / "modes.yaml").read_text())
 
@@ -331,6 +341,12 @@ def main():
     styling = load_styling()
     area_geom = load_area()
 
+    south_bound = load_south_bound()
+    if south_bound is not None:
+        north = box(-180, south_bound, 180, 90)
+        area_geom = area_geom.intersection(north)
+        print(f"south bound: clipping area at lat {south_bound}", flush=True)
+
     routes = load_routes(modes, load_exclude())
 
     bus_wl = load_bus_whitelist()