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/rust/README.md

@@ -161,7 +161,7 @@ cargo run -p mxgw-cli -- galaxy discover-hierarchy --endpoint http://localhost:5
 
 ### Browsing lazily
 
-For UI trees or OPC UA bridges, use `browse_children` to walk one level at a
+For UI trees or OPC UA bridges, use `browse_children_raw` to walk one level at a
 time instead of paging the full hierarchy. Pass a default request for root
 objects; subsequent calls set `parent_gobject_id`, `parent_tag_name`, or
 `parent_contained_path`. Filter fields match `discover_hierarchy`. Each response
@@ -172,7 +172,7 @@ request and filter semantics.
 ```rust
 use zb_mom_ww_mxgateway_client::generated::galaxy_repository::v1::BrowseChildrenRequest;
 
-let reply = galaxy.browse_children(BrowseChildrenRequest::default()).await?.into_inner();
+let reply = galaxy.browse_children_raw(BrowseChildrenRequest::default()).await?;
 for (child, has_children) in reply.children.iter().zip(reply.child_has_children.iter()) {
     println!("{} expand={}", child.tag_name, has_children);
 }