Temporal: Add tests for duration rounding edge case near +24h UTC shift

This edge case was discovered thanks to Firefox fuzzing:
https://bugzilla.mozilla.org/show_bug.cgi?id=2029455

See tc39/proposal-temporal#3310

LLM disclosure: I used a bot to make a first draft of these, then
hand-edited them until I was satisfied.

Author: ptomato

test/intl402/Temporal/Duration/prototype/round/skipped-day-collapses-bounding-window.js

@@ -0,0 +1,32 @@
+// 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: >
+  Rounding a negative duration where skipped day collapses bounding window
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const relativeTo = Temporal.ZonedDateTime.from("2012-01-01T12:00:00+14:00[Pacific/Apia]");
+const d = new Temporal.Duration(0, 0, 0, -1);
+TemporalHelpers.assertDuration(
+  d.round({ smallestUnit: "days", relativeTo }),
+  0, 0, 0, -1, 0, 0, 0, 0, 0, 0,
+  "-P1D rounded to days with bounding window spanning a skipped calendar day"
+);
+
+const d2 = new Temporal.Duration(0, 0, 0, 0, -25);
+TemporalHelpers.assertDuration(
+  d2.round({ smallestUnit: "days", relativeTo }),
+  0, 0, 0, -1, 0, 0, 0, 0, 0, 0,
+  "-PT25H rounded to 1 day, bounding window extending into a skipped calendar day"
+);
+
+const d3 = new Temporal.Duration(0, 0, 0, 0, -36);
+TemporalHelpers.assertDuration(
+  d3.round({ smallestUnit: "days", relativeTo }),
+  0, 0, 0, -2, 0, 0, 0, 0, 0, 0,
+  "-PT36H rounded to 2 days, bounding window extending into a skipped calendar day"
+);

test/intl402/Temporal/Duration/prototype/total/skipped-day-collapses-bounding-window.js

@@ -0,0 +1,31 @@
+// 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.total
+description: >
+  Totalling a negative duration where skipped day collapses bounding window
+features: [Temporal]
+---*/
+
+const relativeTo = Temporal.ZonedDateTime.from("2012-01-01T12:00:00+14:00[Pacific/Apia]");
+const d = new Temporal.Duration(0, 0, 0, -1);
+assert.sameValue(
+  d.total({ unit: "days", relativeTo }),
+  -1,
+  "-1 day total in days with bounding window spanning a skipped calendar day"
+);
+
+const d2 = new Temporal.Duration(0, 0, 0, 0, -25);
+assert.sameValue(
+  d2.total({ unit: "days", relativeTo }),
+  -25 / 24,
+  "-25 hours total in days, bounding window extending into a skipped calendar day"
+);
+
+const d3 = new Temporal.Duration(0, 0, 0, 0, -36);
+assert.sameValue(
+  d3.total({ unit: "days", relativeTo }),
+  -1.5,
+  "-36 hours total in days, bounding window extending into a skipped calendar day"
+);

test/intl402/Temporal/ZonedDateTime/prototype/since/skipped-day-collapses-bounding-window.js

@@ -0,0 +1,24 @@
+// 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.zoneddatetime.prototype.since
+description: since() result where skipped day collapses bounding window
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const start = Temporal.ZonedDateTime.from("2012-01-01T12:00:00+14:00[Pacific/Apia]");
+const end = Temporal.ZonedDateTime.from("2011-12-31T12:00:00+14:00[Pacific/Apia]");
+TemporalHelpers.assertDuration(
+  start.since(end, { smallestUnit: "days" }),
+  0, 0, 0, 1, 0, 0, 0, 0, 0, 0,
+  "since spanning a skipped calendar day"
+);
+
+const end2 = Temporal.ZonedDateTime.from("2011-12-31T11:00:00+14:00[Pacific/Apia]");
+TemporalHelpers.assertDuration(
+  start.since(end2, { smallestUnit: "days" }),
+  0, 0, 0, 1, 0, 0, 0, 0, 0, 0,
+  "bounding window extending into a skipped calendar day"
+);

test/intl402/Temporal/ZonedDateTime/prototype/until/skipped-day-collapses-bounding-window.js

@@ -0,0 +1,24 @@
+// 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.zoneddatetime.prototype.until
+description: until() result where skipped day collapses bounding window
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const start = Temporal.ZonedDateTime.from("2012-01-01T12:00:00+14:00[Pacific/Apia]");
+const end = Temporal.ZonedDateTime.from("2011-12-31T12:00:00+14:00[Pacific/Apia]");
+TemporalHelpers.assertDuration(
+  start.until(end, { smallestUnit: "days" }),
+  0, 0, 0, -1, 0, 0, 0, 0, 0, 0,
+  "until spanning a skipped calendar day"
+);
+
+const end2 = Temporal.ZonedDateTime.from("2011-12-31T11:00:00+14:00[Pacific/Apia]");
+TemporalHelpers.assertDuration(
+  start.until(end2, { smallestUnit: "days" }),
+  0, 0, 0, -1, 0, 0, 0, 0, 0, 0,
+  "bounding window extending into a skipped calendar day"
+);