Dynamic topologies

TopologyEventEngine mutates the graph mid-episode: churn (online/offline), migration (subnet/IP), arrival (BYOD from padding slots).

Events: host_offline, host_online, host_migrate, host_arrive.

cfg = {
    'topology_churn_rate': 0.02,
    'topology_migration_rate': 0.01,
    'topology_arrival_rate': 0.005,
}
env = NetForgeRLEnv(cfg)

Rates also scale with Curriculum.