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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -227,8 +227,8 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     /// <inheritdoc />
     public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
     {
-        // #630: InitializeAsync is reachable on a live driver (DriverInstanceActor.AdoptConfigDuringInit runs
-        // it with no teardown first); `Session = session` below would otherwise orphan the previous session,
+        // #630: InitializeAsync is reachable on a live driver with no teardown first (the IDriver contract
+        // allows it, and a reconnect retry can overlap a running attempt); `Session = session` below would otherwise orphan the previous session,
         // its keep-alive handler and any reconnect handler.
         await ReleaseSessionAsync(cancellationToken).ConfigureAwait(false);
         _health = new DriverHealth(DriverState.Initializing, null, null);