Implement worker register and unregister

docs/implementation-plan-mxaccess-worker.md

@@ -189,6 +189,8 @@ Tests:
 
 Labels: `area:worker`, `type:feature`, `priority:p0`
 
+Status: implemented.
+
 Deliverables:
 
 - `Register`,
@@ -447,4 +449,3 @@ Acceptance criteria:
 
 - each public method has planned parity fixture or documented gap,
 - gateway results preserve HRESULT/status/value/event shape.
-

docs/mxaccess-worker-instance-design.md

@@ -294,7 +294,10 @@ creates `LMXProxyServerClass` through `MxAccessComObjectFactory` on the STA,
 attaches `MxAccessBaseEventSink`, and returns `WorkerReady` only after those
 steps succeed. `MxAccessSession` keeps the raw COM object private, records the
 STA managed thread id that created it, detaches the base event sink during
-disposal, and releases the COM reference on the STA.
+disposal, and releases the COM reference on the STA. After creation,
+`MxAccessStaSession` owns a `StaCommandDispatcher` backed by
+`MxAccessCommandExecutor`; `DispatchAsync` queues contract commands back to the
+same STA instead of exposing the COM object to callers.
 
 Creation rules:
 
@@ -414,6 +417,21 @@ Diagnostics:
 Implement method-specific dispatch instead of a generic string method invoker.
 Parity tests need stable command-specific request and reply shapes.
 
+`MxAccessCommandExecutor` implements the first command pair:
+
+- `Register` calls `LMXProxyServerClass.Register` with the requested client
+  name and preserves the returned server handle in both `ReturnValue` and
+  `RegisterReply.ServerHandle`.
+- `Unregister` calls `LMXProxyServerClass.Unregister` with the requested server
+  handle. The reply has no method-specific payload because the public MXAccess
+  method returns `void`.
+
+Both commands set `Hresult` to `0` only after the COM call returns normally.
+COM exceptions flow through `StaCommandDispatcher`, which captures the thrown
+HRESULT and converts the reply to `ProtocolStatusCode.MxaccessFailure`.
+`MxAccessStaSession.GetRegisteredServerHandlesAsync` returns an STA-read
+snapshot of tracked server handles for diagnostics and future cleanup logic.
+
 ## Handle Registry
 
 The worker should track MXAccess state for diagnostics and cleanup, while still
@@ -434,6 +452,8 @@ Rules:
 
 - Do not invent handles.
 - Do not rewrite handles returned by MXAccess.
+- Record server handles only after `Register` succeeds.
+- Remove server handles only after `Unregister` succeeds.
 - Preserve invalid-handle behavior from MXAccess.
 - Preserve cross-server handle behavior from MXAccess.
 - Use registry state for cleanup and diagnostics, not semantic correction.