rename: prefix gateway projects/namespaces with ZB.MOM.WW + sln→slnx

Apply the ZB.MOM.WW. prefix to all gateway-side projects, folders,
.csproj/.sln contents, C# namespaces, using directives, generated proto
C# (csharp_namespace + checked-in generated files), InternalsVisibleTo
attributes, project-name string literals (LoadProject, .sln lookups,
worker exe paths, staticwebassets manifest), and the install/script/doc
references that point at any of the above. Migrate the solution from
.sln to .slnx via `dotnet sln migrate` and delete the old file.

External-runtime identifiers are intentionally NOT prefixed so external
configuration keeps working:
- GatewayMetrics.cs MeterName ("MxGateway.Server")
- DashboardAuthenticationDefaults Scheme/Policy ("MxGateway.Dashboard")
- GatewayRequestLoggingMiddleware logger category ("MxGateway.Request")
- StaRuntime thread name ("MxGateway.Worker.STA")
- appsettings.json root section "MxGateway" + env-var prefix
  MxGateway__... and secret-name MxGateway:ApiKeyPepper
- C:\ProgramData\MxGateway\ data dir paths

Also fixes two tests that were not rename-related but became visible
while validating the rename:

- WorkerLiveMxAccessSmokeTests.ShutDownAsync: cancellation that the
  gateway service correctly maps to RpcException(Cancelled) per gRPC
  convention was being misclassified as a stream fault. Added a sibling
  catch on RpcException with StatusCode.Cancelled.

- IntegrationTestEnvironment.ResolveRepositoryRoot: extracted IsRepositoryRoot
  and made it accept either a .git marker OR a .sln/.slnx next to src/
  so the worker-exe walker works in non-git working copies.

clients/proto/proto-inputs.json's protoRoot updated to point at
src/ZB.MOM.WW.MxGateway.Contracts/Protos.

Verified by `dotnet build` and a full `dotnet test` of the .slnx with
MXGATEWAY_RUN_LIVE_{MXACCESS,LDAP,GALAXY}_TESTS=1:
  Tests: 472/472 pass
  Worker.Tests: 280/280 pass (4 dev-rig [Fact(Skip=...)] skipped)
  IntegrationTests: 18/18 pass

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/Sessions.md

@@ -16,7 +16,7 @@ All four interfaces (`ISessionManager`, `ISessionRegistry`, `ISessionWorkerClien
 
 The session id is an opaque string in the form `session-{guid:N}` and the per-session pipe name is `mxaccess-gateway-{ProcessId}-{SessionId}`. Encoding the gateway PID into the pipe name avoids collisions when an old gateway process leaks pipes that the OS has not yet reclaimed.
 
-`SessionState` itself is the protobuf-generated enum from `MxGateway.Contracts.Proto`, so it is shared between the gateway and clients on the wire.
+`SessionState` itself is the protobuf-generated enum from `ZB.MOM.WW.MxGateway.Contracts.Proto`, so it is shared between the gateway and clients on the wire.
 
 ```csharp
 public void TransitionTo(SessionState nextState)
@@ -33,12 +33,19 @@ public void TransitionTo(SessionState nextState)
             return;
         }
 
+        if (_state is SessionState.Closing
+            && nextState is not SessionState.Closed
+            && nextState is not SessionState.Faulted)
+        {
+            return;
+        }
+
         _state = nextState;
     }
 }
 ```
 
-`Closed` is terminal and `Faulted` only allows a transition to `Closed`. This guards against late callbacks (worker exit, heartbeat timeout) re-animating a session that is already torn down.
+`Closed` is terminal, `Faulted` only allows a transition to `Closed`, and `Closing` only allows a transition to `Closed` or `Faulted`. This guards against late callbacks (worker exit, heartbeat timeout) re-animating a session that is already tearing down or torn down — once `CloseAsync` has set `Closing` under `_syncRoot`, no `TransitionTo(Ready)` from another thread can walk the session back to `Ready`. Both close-related writes (`Closing` and `Closed`) go through `_syncRoot` exactly like every other state write; `_closeLock` only serializes concurrent close attempts.
 
 ### SessionManager (ISessionManager)
 
@@ -184,7 +191,7 @@ Sessions open with `MxGateway:Sessions:DefaultLeaseSeconds` (default 1800) added
 
 ### Close
 
-`GatewaySession.CloseAsync` is serialized by a per-session `SemaphoreSlim` (`_closeLock`). It transitions to `Closing`, asks the worker client to shut down within `ShutdownTimeout`, and on success transitions to `Closed`. If `WorkerClient.ShutdownAsync` throws, the session falls back to `IWorkerClient.Kill` (forced close):
+`GatewaySession.CloseAsync` is serialized by a per-session `SemaphoreSlim` (`_closeLock`) so only one close runs at a time, but every read/write of `_state` still passes through `_syncRoot` (via `TryBeginClose` and `MarkClosed`). The close path therefore obeys the same lock discipline as `TransitionTo` / `MarkFaulted`: it transitions to `Closing`, asks the worker client to shut down within `ShutdownTimeout`, and on success transitions to `Closed`. `DisposeAsync` waits on `_closeLock` once before disposing the semaphore so an in-flight close's `Release()` cannot race against the dispose. If `WorkerClient.ShutdownAsync` throws, the session falls back to `IWorkerClient.Kill` (forced close):
 
 ```csharp
 if (_workerClient is not null)