Add three test files exercising the servertest harness:
- lifecycle_test.go: connection, disconnection, reconnection, session
replacement, and mesh formation at various sizes.
- consistency_test.go: symmetric visibility, consistent peer state,
address presence, concurrent join/leave convergence.
- weather_test.go: rapid reconnects, flapping stability, reconnect
with various delays, concurrent reconnects, and scale tests.
All tests use table-driven patterns with subtests.
