feat(raft): add commit queue, election timers, and peer health tracking (B1+B2+B3)

- CommitQueue<T>: channel-based queue for committed entries awaiting state machine application
- RaftPeerState: tracks replication and health state (nextIndex, matchIndex, lastContact)
- RaftNode: CommitIndex/ProcessedIndex tracking, election timer with randomized 150-300ms interval,
  peer state integration with heartbeat and replication updates
- 52 new tests across RaftApplyQueueTests, RaftElectionTimerTests, RaftHealthTests

src/NATS.Server/Raft/RaftPeerState.cs

@@ -0,0 +1,46 @@
+namespace NATS.Server.Raft;
+
+/// <summary>
+/// Tracks replication and health state for a single RAFT peer.
+/// Go reference: raft.go peer tracking fields (nextIndex, matchIndex, last contact).
+/// </summary>
+public sealed class RaftPeerState
+{
+    /// <summary>
+    /// The peer's unique node identifier.
+    /// </summary>
+    public required string PeerId { get; init; }
+
+    /// <summary>
+    /// The next log index to send to this peer (leader use only).
+    /// </summary>
+    public long NextIndex { get; set; } = 1;
+
+    /// <summary>
+    /// The highest log index known to be replicated on this peer.
+    /// </summary>
+    public long MatchIndex { get; set; }
+
+    /// <summary>
+    /// Timestamp of the last successful communication with this peer.
+    /// </summary>
+    public DateTime LastContact { get; set; } = DateTime.UtcNow;
+
+    /// <summary>
+    /// Whether this peer is considered active in the cluster.
+    /// </summary>
+    public bool Active { get; set; } = true;
+
+    /// <summary>
+    /// Returns true if this peer has been contacted within the election timeout window.
+    /// Go reference: raft.go isCurrent check.
+    /// </summary>
+    public bool IsCurrent(TimeSpan electionTimeout)
+        => DateTime.UtcNow - LastContact < electionTimeout;
+
+    /// <summary>
+    /// Returns true if this peer is both active and has been contacted within the health threshold.
+    /// </summary>
+    public bool IsHealthy(TimeSpan healthThreshold)
+        => Active && DateTime.UtcNow - LastContact < healthThreshold;
+}