Fix swarm advection (order=2) crash after adding particles (underworldcode#108)

add_particles_with_coordinates() called PETSc's dm.migrate() directly
without invalidating the SwarmVariable _canonical_data caches. The RK2
advection buffer (_X0) retained a stale numpy array with the old particle
count, causing a shape mismatch on the next advection call.

Add cache invalidation after migrate, mirroring the existing pattern in
_recalculate_owner_and_migrate().

Underworld development team with AI support from Claude Code (https://claude.com/claude-code)

Author: lmoresi

src/underworld3/swarm.py

@@ -3380,6 +3380,13 @@ def add_particles_with_coordinates(self, coordinatesArray) -> int:
                 self._remeshed.data[...] = 0
 
         self.dm.migrate(remove_sent_points=True)
+
+        # Invalidate cached data — particle count changed after addNPoints + migrate
+        self._particle_coordinates._canonical_data = None
+        for var in self._vars.values():
+            if hasattr(var, "_canonical_data"):
+                var._canonical_data = None
+
         return npoints
 
     @timing.routine_timer_decorator