fix: resolve stale deriveds with latest value (#18167)

While looking into #18162 I found an adjacent bug. Currently, if an
async derived resolves in batch 2 before it resolves in batch 1, we
reject the promise belonging to batch 1 and by extension the batch
itself. This means that any other changes in batch 1 are silently
discarded, incorrectly.

The fix is almost comically simple: rather than rejecting the earlier
promise, we just resolve it with the latest value.

I have a hunch that this might also enable us to simplify the rebase
logic, though I haven't investigated that in this PR.

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/modern-tables-fetch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: resolve stale deriveds with latest value

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -232,7 +232,7 @@ export function async_derived(fn, label, location) {
 				for (const [b, d] of deferreds) {
 					deferreds.delete(b);
 					if (b === batch) break;
-					d.reject(STALE_REACTION);
+					d.resolve(value);
 				}
 
 				if (DEV && location !== undefined) {

packages/svelte/tests/runtime-runes/samples/async-stale-derived-3/_config.js

@@ -0,0 +1,29 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [increment, pop] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>increment</button><button>pop</button><p>0 0 0</p>`
+		);
+
+		pop.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>increment</button><button>pop</button><p>2 2 1</p>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-stale-derived-3/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	let count = $state(0);
+	let other = $state(0);
+
+	const queue = [];
+
+	function push(v) {
+		if (v === 0) return v;
+
+		return new Promise((fulfil) => {
+			queue.push(() => fulfil(v));
+		});
+	}
+</script>
+
+<button onclick={() => {
+	if (count === 0) other++;
+	count++;
+}}>increment</button>
+<button onclick={() => queue.pop()?.()}>pop</button>
+
+<p>{await push(count)} {count} {other}</p>