feat(alarms): thread an additive AckTime through the native-alarm mirror

MES alarm-status API §6.4 (docs/plans/2026-06-30-mes-alarm-status-api.md,
Phase 1 task 1). MES needs a real AckDT for a triggered alarm, and the mirror
carried acked-vs-unacked but never WHEN. AckTime now rides the whole path:
DCL transition -> AlarmStateChanged -> gRPC AlarmStateUpdate -> site SQLite.

Stamping rule, identical on both protocols: non-null ONLY while the condition
is active AND acknowledged. That single predicate yields all three required
behaviours -- null while unacked, cleared on re-raise (a re-raise arrives
unacknowledged), and no phantom ack on a return-to-normal. The last one is
load-bearing for MxGateway, which maps INACTIVE to Acknowledged = true; without
the active check every clear would claim an ack the system never observed.

Provenance is honest, never fabricated:
  - OPC UA A&C supplies a TRUE ack instant, so we now select it:
    AcknowledgeableConditionType/AckedState/TransitionTime at SelectClause
    index 18, APPENDED so the positional reads at 0-17 keep their meaning.
    Servers that omit the field fall back to the event's own Time.
  - MxAccess Gateway supplies none, so the ack transition's own timestamp is
    used -- accurate to when the system SAW the ack. An ACTIVE_ACKED
    re-subscribe snapshot restores one from LastTransitionTimestamp rather
    than dropping it.
The decision lives in pure mappers (Opc/Mx AlarmMapper.DeriveAckTime), so it is
unit-tested with no live server or gateway.

Additive-only throughout: init-only property on AlarmStateChanged, trailing
optional positional on NativeAlarmTransition (all 14-arg call sites untouched),
proto field 24 (never reusing a number) regenerated via docker/regen-proto.sh
sitestream with the csproj diff verified empty.

Persistence rides native_alarm_state's existing metadata_json blob rather than
a new column -- deliberately. That table is RegisterReplicated in
SiteLocalDbSetup and LocalDb builds its CDC triggers from the column list at
registration time, so an additive JSON property changes no schema, no triggers
and no replication contract; metadata_json is exactly the extension point UA4
introduced for this. Rows written before the field deserialize it as null.

Tests: 4 OPC UA + 6 MxGateway mapper cases, 3 NativeAlarmActor (emit,
failover rehydrate, pre-AckTime row), 1 proto round-trip incl. the null case,
4 Commons additive/back-compat. The OPC UA SelectClause count lock-in moves
18 -> 19 with an index-18 assertion -- intended, the clause is appended, which
is precisely what that guard exists to make visible.

src/ZB.MOM.WW.ScadaBridge.Communication/Protos/sitestream.proto

@@ -97,6 +97,12 @@ message AlarmStateUpdate {
   string limit_value = 21;
   string native_source_canonical_name = 22;  // native binding canonical name; empty for computed
   bool is_configured_placeholder = 23;        // true for a quiet-binding placeholder row
+
+  // Ack timestamp for the condition; null while unacknowledged and on computed alarms
+  // (MES alarm-status API §6.4). The site stamps the SOURCE's own ack instant where the
+  // protocol supplies one (OPC UA A&C AckedState/TransitionTime) and the DCL's observation
+  // time of the ack transition where it does not (MxAccess Gateway).
+  google.protobuf.Timestamp ack_time = 24;
 }
 
 // Audit Log (#23) telemetry: single lifecycle event ferried from a site SQLite