Temporal: Add Duration.round() case triggering temporal_rs bug

I'm not sure of the exact circumstances that trigger this bug, but it
seems to be balancing a duration to a larger unit while also rounding to
an increment, with ZonedDateTime relativeTo.

Discovered using snapshot testing.
V8/Boa bug: boa-dev/temporal#689

Author: ptomato

test/built-ins/Temporal/Duration/prototype/round/rounding-increment-relativeto.js

@@ -0,0 +1,21 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.round
+description: Test a specific buggy case from temporal_rs
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const d = new Temporal.Duration(0, 0, 0, 0, 48);
+const plainRelativeTo = new Temporal.PlainDate(2020, 1, 1);
+const zonedRelativeTo = new Temporal.ZonedDateTime(0n, "UTC");
+
+for (const relativeTo of [undefined, plainRelativeTo, zonedRelativeTo]) {
+  TemporalHelpers.assertDuration(d.round({
+    smallestUnit: "days",
+    roundingIncrement: 2,
+    relativeTo
+  }), 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, `with relativeTo ${relativeTo}`);
+}