Test: reproduce unnecessary remount of a retained nested child

[nguyenyou]

Adds a reproducer for an onMountInsert takeover from children <-- containing a and nested child <-- Val(b) to child <-- b.

Current behavior:
The takeover unmounts a, unmounts b, then mounts b again.

Expected behavior:
Only a should unmount. The same b is already mounted under the same DOM parent and should be retained, matching the existing non-nested takeover test and the cleanup helper’s retention contract.

Tests only; no implementation changes. Against master, the existing 13 takeover tests pass and the new reproducer fails.

Run:
sbt 'testOnly *InserterTakeoverSpec'

[raquo]

Thanks, I've extended keepNode logic to support seamless moves in more complicated cases involving inserter nesting.

As part of this I've also added a new minimizeDiff param to ReplaceAll command – if true, it will avoid unnecessary re-mounts. In case of ReplaceAll, this has non-trivial cost, and children.command is intended for performance critical applications, so it's up to the user to choose what they want for their use case – as explained in scaladoc.