fix(IPC-36,IPC-37): guard secondary contracts

docs/Contracts.md

@@ -94,6 +94,15 @@ only and does not share types with `mxaccess_gateway.proto`. See
 [Galaxy Repository Browse](./GalaxyRepository.md) for the RPC catalog and
 behavior.
 
+The retained proto remains the client-generation source of truth while the
+gateway serves the wire-identical descriptor from the
+`ZB.MOM.WW.GalaxyRepository` package. `GalaxyContractParityTests` converts both
+loaded descriptors to `FileDescriptorProto`, removes source locations, and
+normalizes the C# namespace and file name before comparing deterministic bytes.
+Its mutation test also requires drift failures to identify the first differing
+protobuf message. Any parity failure must be resolved before publishing either
+Contracts or clients.
+
 Generated C# output is written to `src/ZB.MOM.WW.MxGateway.Contracts/Generated/`. Do not
 hand-edit generated files.
 
@@ -140,6 +149,13 @@ Run the focused contract tests after changing either `.proto` file:
 dotnet test src/ZB.MOM.WW.MxGateway.Tests/ZB.MOM.WW.MxGateway.Tests.csproj --filter ProtobufContractRoundTripTests
 ```
 
+When changing `galaxy_repository.proto` or the served Galaxy Repository
+package, also run the retained-vs-served parity guard:
+
+```bash
+dotnet test src/ZB.MOM.WW.MxGateway.Tests/ZB.MOM.WW.MxGateway.Tests.csproj --filter GalaxyContractParityTests
+```
+
 The full solution build also regenerates the C# contracts before compiling
 gateway and test projects: