refactor(runtime): delete the dead in-place ApplyDelta seam; config change = respawn only (#665)

Since #516 a DriverConfig change is stop + respawn, so DriverSpawnPlan.ToApplyDelta
was always empty and the in-place path had no producer. #663 and #664 were both
latent bugs in it. This deletes it:
- host: ApplyChildDelta, _pendingDelta, HandleApplyResult and HandleDeltaApplied
- plan: DriverSpawnPlan.ToApplyDelta
- actor: the ApplyDelta, ApplyResult and DeltaApplied messages, the Connected
  reinit handler and the mid-connect AdoptConfigDuringInit
- tests: the actor tests that drove the seam

IDriver.ReinitializeAsync and each driver's implementation and unit tests stay.
Comments and docs that cited AdoptConfigDuringInit as the reason a driver must
release prior state at init now cite the IDriver contract and overlapping
retries, which still apply.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverSpawnPlan.cs

@@ -3,38 +3,31 @@ namespace ZB.MOM.WW.OtOpcUa.Runtime.Drivers;
 /// <summary>
 ///     Pure diff between the currently-running driver children (keyed by
 ///     <c>DriverInstance.DriverInstanceId</c>) and the target spec list from a freshly-applied
-///     deployment artifact. The DriverHostActor consumes the three lists and calls
-///     spawn / ApplyDelta / stop on its child actors accordingly.
+///     deployment artifact. The DriverHostActor consumes the two lists and calls
+///     spawn / stop on its child actors accordingly. There is no in-place config path: since #516 a config
+///     change is a stop + respawn, and #665 deleted the unreachable in-place <c>ApplyDelta</c> seam.
 /// </summary>
-/// <param name="ToSpawn">Specs with no current child — create a new actor.</param>
-/// <param name="ToApplyDelta">
-///     In-place config deltas. <b>Always empty since #516</b> — a DriverConfig change is now a
-///     stop + respawn so the factory is the single parse authority. Retained because
-///     <c>DriverInstanceActor</c> still handles <c>ApplyDelta</c> on its own config-adoption path
-///     (a config arriving mid-connect), and removing the list would hide that seam.
-/// </param>
+/// <param name="ToSpawn">Specs with no current child (or whose child is being replaced) — create a new actor.</param>
 /// <param name="ToStop">DriverInstanceIds currently running but missing from the new artifact, or now disabled.</param>
 public sealed record DriverSpawnPlan(
     IReadOnlyList<DriverInstanceSpec> ToSpawn,
-    IReadOnlyList<DriverInstanceSpec> ToApplyDelta,
     IReadOnlyList<string> ToStop);
 
 public static class DriverSpawnPlanner
 {
     /// <summary>
-    ///     Compute the spawn/delta/stop sets. Disabled entries in <paramref name="target"/> are
+    ///     Compute the spawn/stop sets. Disabled entries in <paramref name="target"/> are
     ///     treated as "not desired here": if a child exists for the id it goes into ToStop,
     ///     otherwise the row is dropped entirely (no spawn for a disabled driver).
     /// </summary>
     /// <param name="current">The currently running driver children keyed by ID.</param>
     /// <param name="target">The target driver instances from the deployment artifact.</param>
-    /// <returns>The computed <see cref="DriverSpawnPlan"/> with spawn, apply-delta, and stop sets.</returns>
+    /// <returns>The computed <see cref="DriverSpawnPlan"/> with spawn and stop sets.</returns>
     public static DriverSpawnPlan Compute(
         IReadOnlyDictionary<string, DriverChildSnapshot> current,
         IReadOnlyList<DriverInstanceSpec> target)
     {
         var toSpawn = new List<DriverInstanceSpec>();
-        var toDelta = new List<DriverInstanceSpec>();
         var toStop = new List<string>();
 
         var targetById = new Dictionary<string, DriverInstanceSpec>(StringComparer.Ordinal);
@@ -83,7 +76,7 @@ public static class DriverSpawnPlanner
             toSpawn.Add(spec);
         }
 
-        return new DriverSpawnPlan(toSpawn, toDelta, toStop);
+        return new DriverSpawnPlan(toSpawn, toStop);
     }
 }