fix: address final ackmsg-as-operator review findings

- AlarmAckWriter: classify AuthenticateUser and AddItem/Advise/Write
  failures by HRESULT for any interop exception (ArgumentException for
  E_INVALIDARG, UnauthorizedAccessException, COMException); server-gone
  HRESULTs at AuthenticateUser tear down the registration and report
  WriteRejected.
- Bound the ack item cache with a 256-entry LRU (eviction best-effort
  UnAdvise+RemoveItem).
- After an Unconfirmed ack, drop and re-add the item before the next
  write so a late OnWriteComplete on the old handle is not credited.
- Pass StaRuntime.PumpPendingMessages to the ack writer through a
  pump-aware alarm handler factory so long completion waits refresh the
  STA watchdog heartbeat.
- Rust CLI: --operator is required for acknowledge-alarm.
- Docs: full ack status table, credential-oracle trade-off, upgrade note.

Claude-Session: https://claude.ai/code/session_016J6isfcgqy4SB2RK3ZvFvT

docs/DesignDecisions.md

@@ -818,6 +818,14 @@ per-request credential is workable. See
 - The managed ack consumer, the WNAL native ack consumer, the second wnwrap
   ack-only consumer and the opt-in managed-ack environment flag are removed.
 
+**Upgrade note.** This is a breaking change for callers. They must now send
+`operator_user` (required; empty is rejected with `InvalidArgument`) and the
+operator's Galaxy password; a caller that sends no password for a
+password-protected account gets `MXACCESS_FAILURE` from `AuthenticateUser`
+instead of an acknowledgement. OtOpcUa's `GalaxyDriver` alarm acknowledger is a
+known caller that must be updated to forward the operator's credentials before
+the gateway is upgraded.
+
 ## Related Documentation
 
 - [Gateway Process Detailed Design](./GatewayProcessDesign.md)