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/Avl/SequenceSet.cs

@@ -0,0 +1,7 @@
+namespace NATS.Server.Internal.Avl;
+
+// Go reference: server/avl/seqset.go
+// TODO: Port AVL-backed sparse sequence set
+public class SequenceSet
+{
+}

src/NATS.Server/Internal/Gsl/GenericSubjectList.cs

@@ -0,0 +1,7 @@
+namespace NATS.Server.Internal.Gsl;
+
+// Go reference: server/gsl/gsl.go
+// TODO: Port generic trie-based subject list
+public class GenericSubjectList<T> where T : IEquatable<T>
+{
+}

src/NATS.Server/Internal/SubjectTree/SubjectTree.cs

@@ -0,0 +1,7 @@
+namespace NATS.Server.Internal.SubjectTree;
+
+// Go reference: server/stree/stree.go
+// TODO: Port Adaptive Radix Tree for per-subject state
+public class SubjectTree<T>
+{
+}

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
+{
+}