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]