fix(redundancy): elect the Primary by cluster age, not by address

RedundancyStateActor derived the driver Primary from ClusterState.RoleLeader("driver").
Akka offers two different notions of a "first" member and they are not the same
one: role leader is the lowest-ADDRESSED Up member (host, then port), while
ClusterSingletonManager places singletons on the OLDEST — lowest up-number.

They agree on a freshly-formed cluster, which is why every existing test passed.
They diverge after any restart: the restarted node re-joins as the youngest while
keeping its address, so if it holds the lower address it becomes role leader while
the singletons stay put. The snapshot would then name a Primary that is not
hosting the work, and every Primary-gated surface follows it — inbound device
writes, native-alarm acks, the fleet-wide alerts emit, and the alarm-history
drain would all enable on the wrong node while the node actually running the
singletons stayed gated off.

BuildSnapshot now selects the oldest Up member carrying the driver role, matching
singleton placement. Leaving members are excluded: a node handing its singletons
over must not be named Primary.

NodeRedundancyState.IsRoleLeaderForDriver is renamed IsDriverPrimary, and
NodeHealthInputs.IsDriverRoleLeader likewise. Keeping the old names would have
left the wire contract asserting a derivation the code no longer uses — the same
drift that made this defect invisible.

Proven by a real two-node cluster rather than a mock. RedundancyPrimaryElectionTests
binds the first-joining node to the HIGHER port, so oldest and lowest-address name
different nodes, and includes a fixture assertion that the divergence actually
occurred — without it the real assertion could pass for the wrong reason. Positive
control: restoring the RoleLeader derivation turns exactly the two election tests
red while the fixture check stays green.

Runtime.Tests 440 passed, ControlPlane.Tests 82, Cluster.Tests 36.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorPrimaryGateTests.cs

@@ -216,7 +216,7 @@ public sealed class DriverHostActorPrimaryGateTests : RuntimeActorTestBase
             {
                 new NodeRedundancyState(TestNode, role,
                     IsClusterLeader: role == RedundancyRole.Primary,
-                    IsRoleLeaderForDriver: role == RedundancyRole.Primary,
+                    IsDriverPrimary: role == RedundancyRole.Primary,
                     AsOfUtc: DateTime.UtcNow),
             },
             CorrelationId.NewId()));