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.MTConnect.Tests/MTConnectDriverLifecycleTests.cs

@@ -265,9 +265,8 @@ public sealed class MTConnectDriverLifecycleTests
     [Fact]
     public async Task Initialize_honours_the_config_json_over_the_constructor_options()
     {
-        // The runtime delivers a config change ONLY through this argument (DriverInstanceActor
-        // ApplyDelta -> ReinitializeAsync(json) on the LIVE instance). Ignoring it would make every
-        // MTConnect config change a silent no-op that still seals the deployment green.
+        // The IDriver contract: the driver serves the config document it is handed, not only its
+        // constructor options — ignoring it would turn an in-place config change into a silent no-op.
         MTConnectDriverOptions? seen = null;
         var driver = new MTConnectDriver(Opts(), "mt1", o =>
         {
@@ -533,7 +532,7 @@ public sealed class MTConnectDriverLifecycleTests
     [Fact]
     public async Task Reinitialize_with_an_unreadable_config_keeps_the_running_driver_serving()
     {
-        // Blast radius: an unparseable config edit fails the DEPLOYMENT (ApplyResult(false)); it must
+        // Blast radius: an unparseable in-place config edit fails the reinitialize call; it must
         // not down a driver that is happily serving its previous, valid configuration.
         var (driver, client) = await InitializedDriverAsync();