Temporal: Add tests for some Temporal.Duration.p.round edge cases

Found these using snapshot testing.

roundingincrement-days-large.js tests a failure mode that can happen if
using floating-points to convert large rounding increments to
nanoseconds.

case-where-relativeto-affects-rounding-mode-half-even.js tests an edge
case that is somewhat surprising, but correct as per the specification.

Author: ptomato

test/built-ins/Temporal/Duration/prototype/round/case-where-relativeto-affects-rounding-mode-half-even.js

@@ -0,0 +1,40 @@
+// 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: >
+  Rare edge case where presence or absence of relativeTo affects the rounding
+  behaviour of rounding mode halfEven
+features: [Temporal]
+includes: [temporalHelpers.js]
+---*/
+
+const plainRelativeTo = new Temporal.PlainDate(1970, 1, 1);
+const zonedRelativeTo = new Temporal.ZonedDateTime(0n, "UTC");
+
+const duration = new Temporal.Duration(0, 0, 0, 3, 12);  // 3 days 12 hours
+const commonOptions = { smallestUnit: "hours", roundingIncrement: 8, roundingMode: "halfEven" };
+
+// 3 days 12 hours is 10.5 increments, so halfEven rounds down to 10 increments
+TemporalHelpers.assertDuration(
+  duration.round(commonOptions),
+  0, 0, 0, 3, 8, 0, 0, 0, 0, 0, // 3 days 8 hours
+  "halfEven rounding is downward with no relativeTo"
+);
+
+// Here also we calculate 10.5 increments
+TemporalHelpers.assertDuration(
+  duration.round({ ...commonOptions, relativeTo: plainRelativeTo }),
+  0, 0, 0, 3, 8, 0, 0, 0, 0, 0, // 3 days 8 hours
+  "halfEven rounding is downward with PlainDate relativeTo"
+);
+
+// Since days can be different lengths when relative to ZonedDateTime, the days
+// are accounted separately. 0 days 12 hours is 1.5 increments, so halfEven
+// rounds up to 2 increments
+TemporalHelpers.assertDuration(
+  duration.round({ ...commonOptions, relativeTo: zonedRelativeTo }),
+  0, 0, 0, 3, 16, 0, 0, 0, 0, 0, // 3 days 16 hours
+  "halfEven rounding is upward with ZonedDateTime relativeTo"
+);

test/built-ins/Temporal/Duration/prototype/round/roundingincrement-days-large.js

@@ -0,0 +1,30 @@
+// 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: Calculation with a large, but not too large, roundingIncrement
+features: [Temporal]
+includes: [temporalHelpers.js]
+---*/
+
+const plainRelativeTo = new Temporal.PlainDate(1970, 1, 1);
+const zonedRelativeTo = new Temporal.ZonedDateTime(0n, "UTC");
+
+const relativeToTests = [
+  [undefined, 'no'],
+  [plainRelativeTo, 'plain'],
+  [zonedRelativeTo, 'zoned'],
+];
+
+const duration1 = new Temporal.Duration(0, 0, 0, 0, 0, 0, 9007199254, 740, 991, 0);
+for (const [relativeTo, descr] of relativeToTests) {
+  const result = duration1.round({ smallestUnit: 'days', roundingIncrement: 1e7, relativeTo });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, `round to 1e7 days with ${descr} relativeTo`);
+}
+
+const duration2 = new Temporal.Duration(0, 0, 0, 1);
+for (const [relativeTo, descr] of relativeToTests) {
+  const result = duration2.round({ smallestUnit: 'days', roundingIncrement: 1e8 - 1, roundingMode: 'ceil', relativeTo });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 99999999, 0, 0, 0, 0, 0, 0, `round to 1e8-1 days with ${descr} relativeTo`);
+}