feat: scaffold namespaces for data structures, FileStore, and RAFT

Add stub source files for Internal/Avl, Internal/SubjectTree, Internal/Gsl,
Internal/TimeHashWheel, Raft/RaftState, and Raft/IRaftNode, plus empty test
directories for all new namespaces and a Concurrency suite directory.

src/NATS.Server/Raft/IRaftNode.cs

@@ -0,0 +1,7 @@
+namespace NATS.Server.Raft;
+
+// Go reference: server/raft.go lines 40-92
+// TODO: Port RaftNode interface
+public interface IRaftNode
+{
+}

src/NATS.Server/Raft/RaftState.cs

@@ -0,0 +1,10 @@
+namespace NATS.Server.Raft;
+
+// Go reference: server/raft.go
+public enum RaftState : byte
+{
+    Follower = 0,
+    Leader = 1,
+    Candidate = 2,
+    Closed = 3
+}