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/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriver.cs

@@ -24,8 +24,9 @@ public interface IDriver
 
     /// <summary>
     ///     Apply a config change in place without tearing down the driver process.
-    ///     Invoked by the v2 <c>DriverInstanceActor</c> when ApplyDelta reports that only this
-    ///     driver's config changed in the new deployment.
+    ///     <para>The runtime does not call this today: since #516 a DriverConfig change is a stop + respawn,
+    ///     and #665 deleted <c>DriverInstanceActor</c>'s in-place <c>ApplyDelta</c> path. Kept as the in-place
+    ///     contract each driver implements and tests.</para>
     /// </summary>
     /// <remarks>
     ///     Per <c>docs/v2/driver-stability.md</c> §"In-process only (Tier A/B)" — Reinitialize is the