fix(atom): prevent listeners from being skipped during notify

[petar-omni]

Snapshot the listeners array before iteration in Node.notify() to prevent skipping listeners when one unsubscribes itself during notification. The previous implementation iterated directly over this.listeners, which could cause subsequent listeners to be missed when earlier listeners mutated the array via unsubscribe.

Steps to reproduce the bug:

• Use resultOnce, adds 1 listener now
• Another listener by someone else is added, so 2 listeners now
• notify() is called, first listener cleans up itself, mutates listeners array on node
• 2nd listener in notify() never called

[tim-smart]

I took a look at this for effect v4 and ended up switch to a Set. Effect-TS/effect-smol#1173

Feel free to do the same, otherwise I'll backport it later.

[petar-omni]

@tim-smart Ok, did a backport

[tim-smart]

Just needs a pnpm changeset