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.FOCAS/FocasDriver.cs

@@ -196,8 +196,8 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     /// <returns>A completed task.</returns>
     public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
     {
-        // #630: release the previous device states' loops + clients BEFORE they are overwritten (the actor's
-        // AdoptConfigDuringInit calls this without a teardown first). Not ShutdownAsync: that also drops _poll
+        // #630: release the previous device states' loops + clients BEFORE they are overwritten (a live driver can be
+        // re-initialised without a teardown first). Not ShutdownAsync: that also drops _poll
         // and subscriptions, which are owned by the shutdown path (the actor re-subscribes after every successful init).
         await ReleaseDeviceStatesAsync().ConfigureAwait(false);
         ResetAlarmHealth();