feat: add quorum check before proposing entries (Gap 8.6)

Add HasQuorum() to RaftNode that counts peers with LastContact within
2 × ElectionTimeoutMaxMs and returns true only when self + current peers
reaches majority. ProposeAsync now throws InvalidOperationException with
"no quorum" when HasQuorum() returns false, preventing a partitioned
leader from diverging the log. Add 14 tests in RaftQuorumCheckTests.cs
covering single-node, 3-node, 5-node, boundary window, and heartbeat
restore scenarios. Update RaftHealthTests.LastContact_updates_on_successful_replication
to avoid triggering the new quorum guard.

src/NATS.Server/Raft/RaftLog.cs

@@ -19,6 +19,20 @@ public sealed class RaftLog
         return entry;
     }
 
+    /// <summary>
+    /// Appends an entry with an explicit timestamp. Used in tests and by the
+    /// <see cref="CompactionPolicy.ByAge"/> policy to set controlled creation times.
+    /// </summary>
+    public RaftLogEntry AppendWithTimestamp(int term, string command, DateTime timestamp)
+    {
+        var entry = new RaftLogEntry(_baseIndex + _entries.Count + 1, term, command)
+        {
+            Timestamp = timestamp,
+        };
+        _entries.Add(entry);
+        return entry;
+    }
+
     public void AppendReplicated(RaftLogEntry entry)
     {
         if (_entries.Any(e => e.Index == entry.Index))
@@ -79,4 +93,15 @@ public sealed class RaftLog
     }
 }
 
-public sealed record RaftLogEntry(long Index, int Term, string Command);
+/// <summary>
+/// A single RAFT log entry. Timestamp records when the entry was created (UTC) and
+/// is used by the <see cref="CompactionPolicy.ByAge"/> compaction policy.
+/// </summary>
+public sealed record RaftLogEntry(long Index, int Term, string Command)
+{
+    /// <summary>
+    /// UTC creation time. Defaults to <see cref="DateTime.UtcNow"/> at append time.
+    /// Stored as a JSON-serializable property so it survives persistence round-trips.
+    /// </summary>
+    public DateTime Timestamp { get; init; } = DateTime.UtcNow;
+}