Migrate from openark/raft fork to upstream hashicorp/raft v1.7

[renecannao]

Raft Upstream Migration

Based on the evaluation in PR #59 (docs/raft-evaluation.md), migrate from the 2017 openark/raft fork to upstream hashicorp/raft v1.7.x.

Why

• 9 years of missing security patches, bug fixes, and performance improvements
• Fork uses deprecated APIs removed from upstream
• No pre-vote protocol (split-brain risk)
• No leadership transfer support

Scope (from evaluation)

10 divergent API calls to migrate:

1. raft.NewRaft() — 7-arg → 6-arg constructor (PeerStore removed)
2. raft.Configuration / bootstrapping — replace StaticPeers + EnableSingleNode
3. AddPeer / RemovePeer — replace with AddVoter / RemoveServer
4. Yield() — replace with LeadershipTransfer()
5. SnapshotStore.Create() — signature change (SnapshotMeta.Peers → Configuration)
6. PeerStore — removed entirely, use GetConfiguration()

Estimated effort: ~32 hours (3-5 working days)

Files to modify

• go/raft/raft.go — main initialization (heaviest changes)
• go/raft/file_snapshot.go — snapshot store
• go/raft/fsm.go — FSM (minor)
• go/raft/store.go — compatible, minimal changes
• go/raft/rel_store.go — compatible
• go/raft/http_client.go — transport changes
• go.mod — remove replace directive, add hashicorp/raft v1.7.x