feat(routes): add pool accounting per account and S2 compression codec (D2+D3)

D2: Add FNV-1a-based ComputeRoutePoolIdx to RouteManager matching Go's
route.go:533-545, with PoolIndex on RouteConnection and account-aware
ForwardRoutedMessageAsync that routes to the correct pool connection.

D3: Replace DeflateStream with IronSnappy in RouteCompressionCodec, add
RouteCompressionLevel enum, NegotiateCompression, and IsCompressed
detection. 17 new tests (6 pool + 11 compression), all passing.

src/NATS.Server/NatsServer.cs

@@ -54,6 +54,12 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly TlsRateLimiter? _tlsRateLimiter;
     private readonly SubjectTransform[] _subjectTransforms;
     private readonly RouteManager? _routeManager;
+
+    /// <summary>
+    /// Exposes the route manager for testing. Internal — visible to test project
+    /// via InternalsVisibleTo.
+    /// </summary>
+    internal RouteManager? RouteManager => _routeManager;
     private readonly GatewayManager? _gatewayManager;
     private readonly LeafNodeManager? _leafNodeManager;
     private readonly InternalClient? _jetStreamInternalClient;