fix: ensure reactivity system remains consistent with removals

[trueadm]

Fixes #13002.

There were actually a few issues that needed resolving:

• we weren't correctly disconnecting effects from the parents effects (if an effect is created in a derived, and that derived is in a branch, then the child effect will go to the branch)
• we had a linked list on deriveds for ownership of effects, which was pointless as it was also the same linked list used on effects – meaning a deriveds child would incorrectly then link to when to the effect graph – causing all sorts of random things being destroyed. I have no idea how this went missed, but we just now use a single array called children to store any effects or deriveds that are owned by a derived.
• when we execute effects we need to ensure that the child is the latest after updating deriveds

[changeset-bot]

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