Resolve Client.Rust-021

Client.Rust-021 (Design adherence): RustClientDesign.md "Crate layout"
section now describes the actual flat workspace structure instead of
the aspirational nested form. The replacement text states that the
workspace root is clients/rust/, the top-level crate
zb-mom-ww-mxgateway-client is declared in clients/rust/Cargo.toml
directly, and crates/mxgw-cli/ is the sole [workspace.members] entry.
The accompanying tree lists the real files on disk (Cargo.toml,
Cargo.lock, build.rs, README.md, RustClientDesign.md,
src/{lib,client,session,galaxy,options,auth,error,value,version,generated}.rs
plus the src/generated/ tonic-build output dir, tests/, and
crates/mxgw-cli/).

Doc-only change. cargo build --workspace + cargo test --workspace clean.

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

clients/rust/RustClientDesign.md

@@ -11,25 +11,38 @@ generated contract inputs.
 
 ## Crate Layout
 
-Recommended layout:
+The workspace is rooted at `clients/rust/`. The top-level crate
+`zb-mom-ww-mxgateway-client` is declared by `clients/rust/Cargo.toml` itself
+(flat layout — its `src/` sits directly under the workspace root, not nested
+inside `crates/`). The only `[workspace.members]` entry is the `mxgw-cli`
+binary subcrate under `crates/mxgw-cli/`.
 
 ```text
 clients/rust/
-  Cargo.toml
-  build.rs
-  crates/
-    zb-mom-ww-mxgateway-client/
-      src/lib.rs
-      src/client.rs
-      src/session.rs
-      src/options.rs
-      src/auth.rs
-      src/value.rs
-      src/error.rs
-      src/generated/
-    mxgw-cli/
-      src/main.rs
+  Cargo.toml          # workspace root + top-level crate `zb-mom-ww-mxgateway-client`
+  Cargo.lock
+  build.rs            # tonic-build proto generation
+  README.md
+  RustClientDesign.md
+  src/
+    lib.rs
+    client.rs
+    session.rs
+    galaxy.rs
+    options.rs
+    auth.rs
+    error.rs
+    value.rs
+    version.rs
+    generated.rs      # `pub mod` wrappers around files under `src/generated/`
+    generated/        # tonic-build output (not hand-edited)
   tests/
+    client_behavior.rs
+    proto_fixtures.rs
+  crates/
+    mxgw-cli/         # sole workspace member — binary `mxgw`
+      Cargo.toml
+      src/main.rs
 ```
 
 Expected dependencies: