Prevent duplicate shared pool initialization

[mayuki]

Summary

• publish LogicLooperPool.Shared exactly once with Interlocked.CompareExchange
• reject repeated initialization and dispose pools that lose a concurrent initialization race
• use Volatile.Read when accessing the shared pool
• add regression coverage for sequential and concurrent duplicate initialization

Testing

• dotnet test LogicLooper.sln --nologo --no-restore (68 passed)