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.Modbus.Tests/ModbusLifecycleHygieneTests.cs

@@ -225,9 +225,8 @@ public sealed class ModbusLifecycleHygieneTests
     // -------------------- Re-initialize transport hygiene --------------------
 
     /// <summary>
-    ///     A second <c>InitializeAsync</c> on a live driver — the shape
-    ///     <c>DriverInstanceActor.AdoptConfigDuringInit</c> produces when an ApplyDelta lands while the
-    ///     driver is Connecting/Reconnecting — must dispose the transport the previous initialize
+    ///     A second <c>InitializeAsync</c> on a live driver with no teardown between (the <c>IDriver</c>
+    ///     contract allows it) must dispose the transport the previous initialize
     ///     published before publishing a new one. Pre-fix <c>_transport</c> was simply overwritten,
     ///     stranding a connected transport no later dispose could reach.
     /// </summary>
@@ -251,7 +250,7 @@ public sealed class ModbusLifecycleHygieneTests
         });
 
         await drv.InitializeAsync("{}", CancellationToken.None);
-        // No ShutdownAsync in between — exactly what AdoptConfigDuringInit does.
+        // No ShutdownAsync in between.
         await drv.InitializeAsync("{}", CancellationToken.None);
 
         handed.Count.ShouldBe(2, "each initialize must build its own transport");
@@ -415,8 +414,8 @@ public sealed class ModbusLifecycleHygieneTests
 
     /// <summary>
     ///     Two initializes racing on the same driver — the shape <c>DriverInstanceActor</c> produces by
-    ///     dispatching initialize on a fire-and-forget <c>Task.Run</c> with <c>AdoptConfigDuringInit</c>
-    ///     firing twice — must serialize on the lifecycle gate. Ungated, the teardown at the top of one
+    ///     dispatching initialize on a fire-and-forget <c>Task.Run</c>, where a reconnect retry tick can
+    ///     overlap a still-running attempt — must serialize on the lifecycle gate. Ungated, the teardown at the top of one
     ///     initialize disposes the transport the other just published (or nulls <c>_transport</c>
     ///     mid-connect), leaving the driver holding a disposed transport.
     /// </summary>