Add _accountRoutes ConcurrentDictionary to RouteManager with full
CRUD API: RegisterAccountRoute, UnregisterAccountRoute,
GetDedicatedAccountRoute, HasDedicatedRoute,
GetAccountsWithDedicatedRoutes, and DedicatedRouteCount property.
Update GetRouteForAccount to check dedicated routes before falling
back to pool-based selection. Add 10 unit tests in AccountRouteTests.
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.
