feat(hosts): surface per-host connectivity on /hosts; drop the unwritable table (#521)

`IHostConnectivityProbe` was a dead surface: eleven drivers implement it, `GetHostStatuses()`
had ZERO production call sites, and `OnHostStatusChanged` had no subscriber outside the Galaxy
driver's own aggregator. Per-host connectivity was computed by every driver and read by nobody.

The issue offered "build the publisher or delete it". The publisher as its entity doc described
it — driver nodes upserting `DriverHostStatus` rows — is not buildable: per-cluster mesh Phase 4
gates `AddOtOpcUaConfigDb` on the `admin` role, so a driver-only node has no ConfigDb connection
to write rows with. So the capability is kept and the transport changed.

`DriverHealthChanged.HostStatuses` now carries the probe result to `/hosts` as a Hosts column.
That channel already reached the page, already survives the mesh split via the Phase 5 gRPC
telemetry stream, and already replays a last-value snapshot on re-subscribe — so per-host state
re-primes after a reconnect without a durable store. Both halves of the interface finally do
what they are for: the event triggers a prompt publish, the pull is the source of truth.

The point of the column is the case the driver-level state chip structurally cannot express: a
multi-device driver stays aggregate-Healthy while ONE of its devices is unreachable.

Two traps, both pinned by tests that were falsified against the prod code:

- The host digest MUST be in the publish fingerprint. On a single-host-down transition every
  other fingerprint component is unchanged, so the dedup would swallow exactly the publish
  carrying the news — the trap that already bit the rediscovery signal. Removing it turns the
  guard test red, verified.
- null (no probe) must stay distinct from empty (probe with no hosts). proto3 cannot tell an
  absent repeated field from an empty one, hence the explicit `has_host_statuses` flag;
  collapsing them would render every probe-less driver as one whose devices are all fine.

Dropped: the DriverHostStatus entity, enum, DbSet, model config and table (migration
DropDriverHostStatusTable — empty on every deployment, so the scaffolder's data-loss warning is
moot, and Down() recreates it exactly).

Found en route, NOT fixed here: `DriverInstanceResilienceStatus` is the identical defect — no
writer, no reader, only a DbSet declaration, while the live data rides the
`driver-resilience-status` telemetry channel. Its doc-comment now states that rather than
describing the sampler and AdminUI join that were never built. Filed as #524 rather than widening
this schema change beyond what was asked.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverHealthPublisher.cs

@@ -22,13 +22,20 @@ public interface IDriverHealthPublisher
     /// </param>
     /// <param name="rediscoveryReason">The driver-supplied reason from that event; null when
     ///     <paramref name="rediscoveryNeededUtc"/> is null.</param>
+    /// <param name="hostStatuses">
+    ///     Per-host connectivity from <see cref="IHostConnectivityProbe.GetHostStatuses"/>, or null when the
+    ///     driver is not an <see cref="IHostConnectivityProbe"/>. Lets a multi-device driver (a FOCAS or
+    ///     AbLegacy instance owning several PLCs) surface ONE unreachable device that would otherwise be
+    ///     invisible behind an aggregate-Healthy driver row.
+    /// </param>
     void Publish(
         string clusterId,
         string driverInstanceId,
         DriverHealth health,
         int errorCount5Min,
         DateTime? rediscoveryNeededUtc = null,
-        string? rediscoveryReason = null);
+        string? rediscoveryReason = null,
+        IReadOnlyList<HostConnectivityStatus>? hostStatuses = null);
 }
 
 /// <summary>
@@ -49,6 +56,7 @@ public sealed class NullDriverHealthPublisher : IDriverHealthPublisher
         DriverHealth health,
         int errorCount5Min,
         DateTime? rediscoveryNeededUtc = null,
-        string? rediscoveryReason = null)
+        string? rediscoveryReason = null,
+        IReadOnlyList<HostConnectivityStatus>? hostStatuses = null)
     { /* no-op */ }
 }