Add TraceContextPropagator and TraceContext to Internal/MessageTraceContext.cs
for Nats-Trace-Parent header injection and extraction across server hops.
Also add ConnzSortOption/ConnzSorter stubs to fix pre-existing build errors
in Monitoring/Connz.cs. Covered by 10 new tests in TraceContextPropagationTests.cs.
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.
