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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasReinitializeReleasesStateTests.cs

@@ -5,8 +5,8 @@ using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
 namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
 
 /// <summary>
-///     #630 — <c>InitializeAsync</c> on a live driver (<c>DriverInstanceActor.AdoptConfigDuringInit</c> runs
-///     it with no teardown first) overwrote <c>_devices[host]</c> and orphaned the previous state's
+///     #630 — <c>InitializeAsync</c> on a live driver with no teardown first (the <c>IDriver</c> contract
+///     allows it) overwrote <c>_devices[host]</c> and orphaned the previous state's
 ///     probe loop + client. It now releases them first.
 /// </summary>
 [Trait("Category", "Unit")]