fix: avoid reconnecting to dependencies after cleanup (#14)

* fix: cancel reconnection when unsubscribing

* test: remove print

* feat: allow factory to return nil

* perf: use `next` iteration

src/__tests__/sync/validate.spec.luau

@@ -18,7 +18,7 @@ return function()
 			nested_atom = { value = atom(0), expected = false },
 		}
 
-		for key, case in cases do
+		for key, case in next, cases do
 			local success, result = pcall(function()
 				validate(case.value, key)
 			end)