feat(cluster): add learner join workflow and CLI

[Aetherance]

Part 6 of the membership stack.

Depends on #5.

Summary

• authorize fresh learners through JoinInfo using committed member identity and address
• bootstrap a joining database without initially making the local node a voter
• force a membership snapshot so a new learner can catch up before promotion
• add kv join flags and kvctl member management commands
• cover restart persistence and real two-node learner catch-up and promotion

Validation

• go test ./...
• go vet ./...
• go test -race ./engine/storage/raft_storage
• learner join scenario repeated 10 times

Review scope

This is the final PR in the membership stack and contains the operator-facing join workflow and CLI.