fix: don't flag stable NaN keys as volatile in each idempotency check

The dev-mode {#each} key-idempotency guard compared the two get_key
results with strict inequality. A key function that deterministically
returns NaN is idempotent, but NaN !== NaN is always true, so it was
wrongly reported as volatile and each_key_volatile was thrown. Use
Object.is so stable NaN keys compare equal while distinct fresh
references are still detected as volatile.

.changeset/each-key-nan.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't flag stable `NaN` keys as volatile in dev `{#each}` idempotency check

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -306,7 +306,7 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 			if (DEV) {
 				// Check that the key function is idempotent (returns the same value when called twice)
 				var key_again = get_key(value, index);
-				if (key !== key_again) {
+				if (!Object.is(key, key_again)) {
 					e.each_key_volatile(String(index), String(key), String(key_again));
 				}
 			}

packages/svelte/tests/runtime-runes/samples/each-key-nan/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+// a deterministic `NaN` key is idempotent (`NaN !== NaN`, but `Object.is(NaN, NaN)`),
+// so the dev-mode key-idempotency check must not throw `each_key_volatile`
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	mode: ['client'],
+
+	html: `<p>NaN</p>`
+});

packages/svelte/tests/runtime-runes/samples/each-key-nan/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let things = $state([{ id: NaN }]);
+</script>
+
+{#each things as thing (thing.id)}
+	<p>{thing.id}</p>
+{/each}