Adds BeginJointConsensus, CommitJointConsensus, and CalculateJointQuorum to
RaftNode per Raft paper Section 4. During a joint configuration transition
quorum requires majority from BOTH Cold and Cnew; CommitJointConsensus
finalizes Cnew as the sole active configuration. The existing single-phase
ProposeAddPeerAsync/ProposeRemovePeerAsync are unchanged. Includes 16 new
tests covering flag behaviour, quorum boundaries, idempotent commit, and
backward-compatibility with the existing membership API.
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