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.S7.Tests/S7DriverReconnectTests.cs

@@ -483,8 +483,8 @@ public sealed class S7DriverReconnectTests
     }
 
     /// <summary>
-    ///     #630 — a second <c>InitializeAsync</c> on a live driver (no <c>ShutdownAsync</c> between, as
-    ///     <c>DriverInstanceActor.AdoptConfigDuringInit</c> does) must stop the first probe loop and
+    ///     #630 — a second <c>InitializeAsync</c> on a live driver (no <c>ShutdownAsync</c> between — the
+    ///     <c>IDriver</c> contract allows it) must stop the first probe loop and
     ///     dispose the first connection rather than orphan them.
     /// </summary>
     [Fact]