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.Modbus/ModbusDriver.cs

@@ -95,7 +95,7 @@ public sealed class ModbusDriver
     ///     concurrent operations would dispose each other's freshly-built transport or null
     ///     <c>_transport</c> out from under an in-flight connect. Not hypothetical:
     ///     <c>DriverInstanceActor</c> dispatches initialize on a fire-and-forget <c>Task.Run</c> and
-    ///     its <c>AdoptConfigDuringInit</c> can fire twice back to back. Same shape as
+    ///     a reconnect retry tick can start a second attempt while the first is still running. Same shape as
     ///     <c>MqttDriver._lifecycleGate</c>.
     /// </summary>
     private readonly SemaphoreSlim _lifecycleGate = new(1, 1);
@@ -297,9 +297,9 @@ public sealed class ModbusDriver
             // Release whatever a PREVIOUS initialize published before building the replacements. All
             // three fields assigned below (_transport, _probeCts, _reprobeCts) were simply overwritten,
             // so a second initialize on a live driver stranded a connected transport plus two running
-            // background loops that no later dispose could reach. Reachable without any teardown of
-            // ours: DriverInstanceActor re-runs InitializeAsync with no ShutdownAsync when an ApplyDelta
-            // lands while the driver is in Connecting/Reconnecting (AdoptConfigDuringInit).
+            // background loops that no later dispose could reach. The IDriver contract lets a
+            // caller re-initialise a live driver without ShutdownAsync, and DriverInstanceActor's retry tick
+            // can overlap a still-running attempt, so this must not rely on a teardown having happened.
             // Deliberately NOT the shared TeardownAsync(): that also disposes _poll, which tears down
             // every ACTIVE SUBSCRIPTION — and nothing re-subscribes them afterwards, because a
             // mid-life re-initialize is not a shutdown the OPC UA layer knows to re-drive. A caller