proto: add alarm-transition event family + ack/query RPCs (PR A.1)

First PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md in lmxopcua). Pure contract-surface
change — no functional wiring yet. Worker-side subscription (A.2),
gateway-side dispatch + ack handler (A.3), and ConditionRefresh
(A.4) follow.

mxaccess_gateway.proto:

- Extend MxEventFamily with MX_EVENT_FAMILY_ON_ALARM_TRANSITION = 5.
- Extend MxEvent.body oneof with OnAlarmTransitionEvent on_alarm_transition = 24.
- Add OnAlarmTransitionEvent message carrying the full MxAccess alarm
  payload (full reference, source object, alarm-type-name, transition
  kind, raw severity, original raise timestamp, transition timestamp,
  operator user/comment, category, description, current/limit value).
  Mapping to OPC UA 0-1000 severity ladder happens server-side in
  lmxopcua's MxAccessSeverityMapper (B.1) — gateway preserves the
  native MxAccess scale.
- Add AlarmTransitionKind enum (Raise / Acknowledge / Clear / Retrigger).
- Add ActiveAlarmSnapshot + AlarmConditionState for the
  ConditionRefresh stream.
- Add public RPCs AcknowledgeAlarm (unary) and QueryActiveAlarms
  (server-streaming) on MxAccessGateway service.
- Add AcknowledgeAlarmRequest/Reply + QueryActiveAlarmsRequest.

GatewayContractInfo.GatewayProtocolVersion bumps 2 -> 3. Fixture
manifests (proto-inputs, behavior, parity, golden OpenSessionReply)
and protoset descriptor regenerated.

Tests: round-trip serialization for the new messages with
all-fields-populated and empty-optional-fields cases; oneof
last-write-wins guard between OnDataChange and OnAlarmTransition;
descriptor service-method enumeration includes the two new RPCs.
All 273 existing tests still pass.

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

clients/proto/fixtures/behavior/manifest.json

@@ -2,7 +2,7 @@
   "schemaVersion": 1,
   "fixtureSet": "mxaccess-gateway-client-behavior",
   "contractName": "mxaccess-gateway",
-  "gatewayProtocolVersion": 2,
+  "gatewayProtocolVersion": 3,
   "workerProtocolVersion": 1,
   "protoInputManifest": "clients/proto/proto-inputs.json",
   "fixtures": [

clients/proto/fixtures/golden/open-session-reply.ok.json

@@ -3,12 +3,14 @@
   "backendName": "mxaccess-worker",
   "workerProcessId": 1234,
   "workerProtocolVersion": 1,
-  "gatewayProtocolVersion": 2,
+  "gatewayProtocolVersion": 3,
   "capabilities": [
     "unary-open-session",
     "unary-close-session",
     "unary-invoke",
-    "server-stream-events"
+    "server-stream-events",
+    "unary-acknowledge-alarm",
+    "server-stream-active-alarms"
   ],
   "defaultCommandTimeout": "30s",
   "protocolStatus": {

clients/proto/fixtures/parity/parity-fixture-matrix.json

@@ -2,7 +2,7 @@
   "schemaVersion": 1,
   "fixtureSet": "mxaccess-gateway-parity-fixture-matrix",
   "contractName": "mxaccess-gateway",
-  "gatewayProtocolVersion": 2,
+  "gatewayProtocolVersion": 3,
   "workerProtocolVersion": 1,
   "sourceCaptureRoot": "C:/Users/dohertj2/Desktop/mxaccess/captures",
   "sourceDocs": [

clients/proto/proto-inputs.json

@@ -1,7 +1,7 @@
 {
   "schemaVersion": 1,
   "contractName": "mxaccess-gateway",
-  "gatewayProtocolVersion": 2,
+  "gatewayProtocolVersion": 3,
   "workerProtocolVersion": 1,
   "protoRoot": "src/MxGateway.Contracts/Protos",
   "sourceFiles": [