feat(runtime): F7 spawn lifecycle + F20 ShouldStub gate

DriverHostActor.ApplyAndAck now reads the deployment artifact and
reconciles its set of DriverInstanceActor children — spawn the missing,
ApplyDelta to those with changed config, stop the removed/disabled.
The diff lives in pure DriverSpawnPlanner so it can be unit-tested
without an ActorSystem.

Adds IDriverFactory in Core.Abstractions (consumed by Runtime) +
DriverFactoryRegistryAdapter in Core.Hosting that wraps the existing
v1 DriverFactoryRegistry — Runtime stays decoupled from Polly/Serilog,
the Host wires the adapter once driver assemblies have registered.

ShouldStub(type, roles) is now actually called on every spawn — Galaxy
+ Wonderware-Historian boot stubbed on macOS/Linux or whenever the host
carries the dev role. Missing factory ⇒ stub fallback, never a crash.

Tests: 24 → 34 in Runtime (+10):
- DriverSpawnPlannerTests x7 (diff cases, type change ⇒ stop+respawn)
- DeploymentArtifactTests  x5 (empty/malformed/missing fields tolerant)
- DriverHostActorReconcileTests x4 (spawn count, stub fallback,
  ShouldStub gate, second-apply stops the removed)
All 6 v2 test suites green: 120 tests passing.

Closes F20 (ShouldStub wired). F7 marked partial — subscription
publishing + write path still stubbed in DriverInstanceActor itself.

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

@@ -0,0 +1,35 @@
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+/// <summary>
+///     Abstraction over the process-wide driver registry. Runtime consumes this instead of
+///     <c>DriverFactoryRegistry</c> directly so the Runtime project doesn't pull in
+///     <c>ZB.MOM.WW.OtOpcUa.Core</c> (which would drag in Polly + driver hosting). The fused
+///     Host binds a <c>DriverFactoryRegistryAdapter</c> after every <c>Driver.*.Register()</c>
+///     extension has run.
+/// </summary>
+public interface IDriverFactory
+{
+    /// <summary>
+    ///     Return a new <see cref="IDriver"/> for the given <paramref name="driverType"/>, or
+    ///     <c>null</c> when no factory is registered for that type (missing assembly, typo, etc.).
+    ///     The DriverHostActor logs + skips the row rather than failing the whole apply.
+    /// </summary>
+    IDriver? TryCreate(string driverType, string driverInstanceId, string driverConfigJson);
+
+    /// <summary>Driver-type names this factory can materialise. Mostly for diagnostics + logs.</summary>
+    IReadOnlyCollection<string> SupportedTypes { get; }
+}
+
+/// <summary>
+///     Returns <c>null</c> from every <see cref="IDriverFactory.TryCreate"/> call. Bound when the
+///     fused Host hasn't registered any concrete driver assemblies yet (Mac dev path, smoke
+///     tests). DriverHostActor sees zero supported types and treats the deployment as a no-op.
+/// </summary>
+public sealed class NullDriverFactory : IDriverFactory
+{
+    public static readonly NullDriverFactory Instance = new();
+    private NullDriverFactory() { }
+
+    public IDriver? TryCreate(string driverType, string driverInstanceId, string driverConfigJson) => null;
+    public IReadOnlyCollection<string> SupportedTypes { get; } = Array.Empty<string>();
+}