Add NegotiatePoolSize static method and NegotiatedPoolSize property to
RouteConnection, and ConfiguredPoolSize / GetEffectivePoolSize to
RouteManager. Includes 14 tests covering negotiation semantics, backward
compatibility (zero means no pooling), default state, and deterministic
pool index computation.
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.
