feat: implement raft election and term state

src/NATS.Server/Raft/RaftNode.cs

@@ -0,0 +1,56 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftNode
+{
+    private int _votesReceived;
+
+    public string Id { get; }
+    public int Term => TermState.CurrentTerm;
+    public RaftRole Role { get; private set; } = RaftRole.Follower;
+    public RaftTermState TermState { get; } = new();
+    public long AppliedIndex { get; set; }
+
+    public RaftNode(string id)
+    {
+        Id = id;
+    }
+
+    public void StartElection(int clusterSize)
+    {
+        Role = RaftRole.Candidate;
+        TermState.CurrentTerm++;
+        TermState.VotedFor = Id;
+        _votesReceived = 1;
+        TryBecomeLeader(clusterSize);
+    }
+
+    public VoteResponse GrantVote(int term)
+    {
+        if (term < TermState.CurrentTerm)
+            return new VoteResponse { Granted = false };
+
+        TermState.CurrentTerm = term;
+        return new VoteResponse { Granted = true };
+    }
+
+    public void ReceiveVote(VoteResponse response, int clusterSize = 3)
+    {
+        if (!response.Granted)
+            return;
+
+        _votesReceived++;
+        TryBecomeLeader(clusterSize);
+    }
+
+    private void TryBecomeLeader(int clusterSize)
+    {
+        var quorum = (clusterSize / 2) + 1;
+        if (_votesReceived >= quorum)
+            Role = RaftRole.Leader;
+    }
+
+    public void RequestStepDown()
+    {
+        Role = RaftRole.Follower;
+    }
+}