Fix all MxGateway.Client.Rust code-review findings

Resolves Client.Rust-001 through Client.Rust-011.

Build/test/clippy gate (Client.Rust-001/002/003):
- options.rs: doc comments on with_max_grpc_message_bytes /
  max_grpc_message_bytes (#![warn(missing_docs)])
- session.rs: rename BulkReplyKind variants to drop the shared `Bulk`
  suffix (clippy::enum_variant_names)
- galaxy.rs: deref instead of clone on Option<Timestamp>
  (clippy::clone_on_copy — an extra violation the gate also hit)
- mxgw-cli: assert version_json against GATEWAY/WORKER_PROTOCOL_VERSION
  constants instead of the stale literal 2
`cargo clippy --workspace --all-targets -- -D warnings` now passes.

Correctness / error handling:
- version.rs: CLIENT_VERSION = env!("CARGO_PKG_VERSION") (Client.Rust-004)
- session.rs: register/add_item/add_item2 handle extractors and
  bulk_results now return Err(Error::MalformedReply) instead of a
  silent 0 / empty vec on a shapeless OK reply (Client.Rust-005/006)
- error.rs: new Error::Unavailable classifies Code::Unavailable /
  ResourceExhausted as transient (Client.Rust-010)
- session.rs: per-call unique correlation ids via an atomic counter
  (Client.Rust-011)

Other:
- value.rs: MxValue/MxArrayValue compute the projection on demand
  instead of caching it, so a wire-only value pays no projection cost
  (Client.Rust-008)
- RustClientDesign.md: correct the crate layout, drop the unused
  `tracing` dependency (Client.Rust-007)
- client_behavior.rs: tests for the bulk-size cap, a mid-stream status
  fault, and the unreadable-CA-file path (Client.Rust-009)

cargo fmt / test --workspace (27 tests) / clippy all pass.

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

clients/rust/RustClientDesign.md

@@ -11,28 +11,34 @@ generated contract inputs.
 
 ## Crate Layout
 
-Recommended layout:
+Actual layout — the `mxgateway-client` library crate is the workspace root,
+with the `mxgw` test CLI as a workspace member:
 
 ```text
-clients/rust/
+clients/rust/                 # `mxgateway-client` library crate (workspace root)
   Cargo.toml
   build.rs
+  src/
+    lib.rs
+    client.rs
+    session.rs
+    galaxy.rs
+    options.rs
+    auth.rs
+    value.rs
+    version.rs
+    error.rs
+    generated.rs
   crates/
-    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/
+    mxgw-cli/                 # `mxgw` test CLI (workspace member)
+      Cargo.toml
       src/main.rs
   tests/
+    client_behavior.rs
+    proto_fixtures.rs
 ```
 
-Expected dependencies:
+Dependencies:
 
 - `tonic`
 - `prost`
@@ -43,7 +49,6 @@ Expected dependencies:
 - `clap`
 - `serde`
 - `serde_json`
-- `tracing`
 
 ## Library API