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/Internal/TimeHashWheel/HashWheel.cs

@@ -0,0 +1,7 @@
+namespace NATS.Server.Internal.TimeHashWheel;
+
+// Go reference: server/thw/thw.go
+// TODO: Port time hash wheel for TTL expiration
+public class HashWheel
+{
+}