fix: resolve code-review findings (locally verified)

Server-054/055/056, Contracts-020/021/022, Tests-036/038/039,
IntegrationTests-030/031/032 (+033 deferred to live rig),
Client.Dotnet-026/028/029 (+027 won't-fix), Client.Go-030..034,
Client.Python-032..036, Client.Rust-033..038.

Key fix: SessionEventDistributor orphaned a subscriber that registered after
the pump completed but before disposal (Server-056) -> register paths now
complete late registrants under _lifecycleLock; regression test added. The
racy dashboard-mirror gRPC test made deterministic (Tests-039).

Verified green locally: gateway Tests targeted classes (GatewaySession,
SessionEventDistributor, GatewayOptionsValidator, ProtobufContractRoundTrip,
GatewaySessionDashboardMirror) + dotnet/go/python/rust client suites.

clients/python/src/zb_mom_ww_mxgateway/__init__.py

@@ -2,7 +2,7 @@
 
 from .auth import ApiKey, auth_metadata
 from .client import GatewayClient
-from .galaxy import GalaxyRepositoryClient
+from .galaxy import GalaxyRepositoryClient, LazyBrowseNode
 from .generated.galaxy_repository_pb2 import (
     DeployEvent,
     GalaxyAttribute,
@@ -19,19 +19,21 @@ from .errors import (
     MxGatewayTransportError,
     MxGatewayWorkerError,
 )
-from .options import ClientOptions
+from .options import BrowseChildrenOptions, ClientOptions
 from .session import Session
 from .values import MxValueView, from_mx_value, to_mx_value
 from .version import __version__
 
 __all__ = [
     "ApiKey",
+    "BrowseChildrenOptions",
     "ClientOptions",
     "DeployEvent",
     "GalaxyAttribute",
     "GalaxyObject",
     "GalaxyRepositoryClient",
     "GatewayClient",
+    "LazyBrowseNode",
     "MxAccessError",
     "MxGatewayAuthenticationError",
     "MxGatewayAuthorizationError",