fix: robust handling of events in spread attributes

[dummdidumm]

When an event listener is removed right before it would be fired, adding a new listener comes too late for the browser and the event is swallowed. The fix is to tweak the spread attributes function to keep using the same object for updates so that we can wrap the event listener to invoke it like attributes[key](evt) instead.
No test because it seems impossible to reproduce in testing environments. This is the reproducible that fails without this fix:

<script lang="ts">
	let x = $state({
		onfocus: (e) => {
			console.log('focused');
		},
		onblur: (e) => {
			console.log('blurred');
		}
	});

	let i = 0;
	function handleFocus(node: HTMLButtonElement) {
		function changeFocused() {
			i++;
			x = {
				onfocus: (e) => {
					console.log('focused', i);
				},
				onblur: (e) => {
					console.log('blurred', i);
				}
			};
		}
		node.addEventListener('focus', changeFocused);
		node.addEventListener('blur', changeFocused);

		return {
			destroy() {
				node.removeEventListener('focus', changeFocused);
				node.removeEventListener('blur', changeFocused);
			}
		};
	}
</script>

<button>button 1</button>
<button use:handleFocus {...x}>button 2</button>

Fixes #11903

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