feat: add randomized election timeout jitter (Gap 8.8)

Add RandomizedElectionTimeout() method to RaftNode returning TimeSpan in
[ElectionTimeoutMinMs, ElectionTimeoutMaxMs) using TotalMilliseconds (not
.Milliseconds component) to prevent synchronized elections after partitions.
Make Random injectable for deterministic testing. Fix SendHeartbeatAsync stub
in NatsRaftTransport and test-local transport implementations to satisfy the
IRaftTransport interface added in Gap 8.7.

src/NATS.Server/Raft/NatsRaftTransport.cs

@@ -217,4 +217,42 @@ public sealed class NatsRaftTransport : IRaftTransport
         _publish(subject, null, wire.Encode());
         return Task.CompletedTask;
     }
+
+    /// <summary>
+    /// Sends heartbeat RPCs to all listed followers over NATS to confirm quorum for
+    /// linearizable reads. Publishes an empty AppendEntry to each follower's heartbeat
+    /// subject and invokes <paramref name="onAck"/> for each that is considered reachable
+    /// (fire-and-forget in this transport; ACK is optimistic).
+    ///
+    /// Go reference: raft.go — leader sends empty AppendEntries to confirm quorum for reads.
+    /// </summary>
+    public Task SendHeartbeatAsync(
+        string leaderId,
+        IReadOnlyList<string> followerIds,
+        int term,
+        Action<string> onAck,
+        CancellationToken ct)
+    {
+        var appendSubject = RaftSubjects.AppendEntry(_groupId);
+
+        foreach (var followerId in followerIds)
+        {
+            // Encode a heartbeat as an empty AppendEntry (no log entries).
+            var wire = new RaftAppendEntryWire(
+                LeaderId: leaderId,
+                Term: (ulong)term,
+                Commit: 0,
+                PrevTerm: 0,
+                PrevIndex: 0,
+                Entries: [],
+                LeaderTerm: (ulong)term);
+
+            _publish(appendSubject, null, wire.Encode());
+
+            // Optimistically acknowledge — a full implementation would await replies.
+            onAck(followerId);
+        }
+
+        return Task.CompletedTask;
+    }
 }