[M5] mxaccess-asb: F25 step 5 — KeepAlive + Read + one-way client ops

Extends AsbClient with one-way operation support (`IsOneWay = true`
in IASBIDataV2) plus the KeepAlive and Read operations.

Client API additions:
* `send_envelope_one_way(env)` — frames in SizedEnvelope, writes,
  returns immediately. No response read. Mirrors WCF's IsOneWay
  semantics for KeepAlive / Disconnect / AuthenticateMe.
* `send_signed_envelope_one_way(action, body, force_hmac)` —
  one-way variant that runs the body through F23's authenticator
  signing path so the ConnectionValidator header is attached.
* `keep_alive()` — sends an empty `KeepAliveRequest` with default
  signing. Used to keep the channel alive past the WCF inactivity
  timeout (30s default at `MxAsbDataClient.cs:683`).
* `read(items)` — sends a signed Read envelope, decodes
  ReadResponse with both Status and Values arrays.

Operations API additions:
* `build_keep_alive_request_body()` — empty wrapper element +
  asb.contracts.messages namespace. Mirror of `AsbContracts.cs:117`
  (`public sealed class KeepAlive : ConnectedRequest;`).
* `ReadResponse { status: Vec<ItemStatus>, values: Vec<RuntimeValue> }`
  per `AsbContracts.cs:169-179`.
* `decode_read_response(body_tokens)` — pulls both ASBIData
  payloads, decodes Status as ItemStatus[], decodes Values via
  `decode_runtime_value_array` (4-byte int32 count + per-element
  `RuntimeValue::decode` from F24).

5 new tests:
* KeepAlive body shape (empty wrapper, correct namespace).
* ReadResponse decoder round-trip with both Status and Values.
* ReadResponse decoder graceful handling when Values is absent
  (returns empty vec).
* End-to-end client::keep_alive — peer drains SizedEnvelope but
  doesn't respond; client returns Ok().
* End-to-end client::read — peer responds with synthetic
  ReadResponse, client recovers Values[0].timestamp_binary == 1234
  and Values[0].status round-trip.

Stubbed for next F25 iterations:
* AsbClient::connect — DH Connect + AuthenticateMe handshake. Needs
  ConnectRequest / ConnectResponse builders (regular WCF XML, not
  the IAsbCustomSerializableType fast-path).
* Write / PublishWriteComplete / CreateSubscription /
  AddMonitoredItems / Publish / Disconnect operation wrappers.

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

rust/crates/mxaccess-asb/src/lib.rs

@@ -25,7 +25,8 @@ pub use envelope::{
     encode_envelope,
 };
 pub use operations::{
-    OperationError, RegisterItemsResponse, UnregisterItemsResponse, build_read_request_body,
-    build_register_items_request_body, build_unregister_items_request_body,
-    collect_asbidata_payloads, decode_register_items_response, decode_unregister_items_response,
+    OperationError, ReadResponse, RegisterItemsResponse, UnregisterItemsResponse,
+    build_keep_alive_request_body, build_read_request_body, build_register_items_request_body,
+    build_unregister_items_request_body, collect_asbidata_payloads, decode_read_response,
+    decode_register_items_response, decode_unregister_items_response,
 };