Fix start_of("day") landing on the previous day when a zone springs forward at midnight

_start_of_day resets to midnight via self.at(0, 0, 0, 0), which propagates the
receiver's fold. On a timezone that springs forward exactly at midnight (e.g.
Chile/Continental, where 2025-09-07 00:00:00 does not exist), reaching that day
through .add(days=1) yields fold=0. For a nonexistent (skipped) local time with
fold=0, the timezone resolver shifts the instant backward, so midnight lands at
23:00 on the previous day and start_of("day") is wrong and non-idempotent.

After resetting to midnight, if the calendar day changed then midnight was
skipped, so re-create the day's first valid instant resolving forward (fold=1).
The guard fires only when midnight actually moved to the previous day, so all
other zones/dates are byte-identical (verified against UTC, America/New_York,
Asia/Tokyo, Europe/London, Europe/Vienna's 02:00 gap, and America/Sao_Paulo's
midnight fall-back). start_of("week") is covered via delegation.

Fixes #915.

Author: youdie006

src/pendulum/datetime.py

@@ -826,7 +826,19 @@ def _start_of_day(self) -> Self:
         """
         Reset the time to 00:00:00.
         """
-        return self.at(0, 0, 0, 0)
+        dt = self.at(0, 0, 0, 0)
+
+        # In zones that spring forward exactly at midnight, 00:00:00 does not
+        # exist. When ``fold`` is 0, resolving that nonexistent time shifts it
+        # backward into the previous day, which is not the start of this day.
+        # Detect that the day changed and rebuild the first valid instant of
+        # the day by resolving forward (fold=1) instead.
+        if dt.day != self.day:
+            dt = self.__class__.create(
+                self.year, self.month, self.day, 0, 0, 0, 0, tz=self.tz, fold=1
+            )
+
+        return dt
 
     def _end_of_day(self) -> Self:
         """

tests/datetime/test_start_end_of.py

@@ -323,3 +323,18 @@ def test_end_of_on_date_after_transition():
     assert d.end_of("day").offset == 3600
     assert d.end_of("month").offset == 3600
     assert d.end_of("year").offset == 3600
+
+
+def test_start_of_day_when_midnight_does_not_exist():
+    # Chile/Continental springs forward exactly at midnight on 2025-09-07,
+    # so 00:00:00 does not exist that day. start_of("day") must resolve to the
+    # first valid instant of the day (01:00:00-03:00), not fall back into the
+    # previous day. See issue #915.
+    d = pendulum.datetime(2025, 9, 6, 0, 0, tz="Chile/Continental").add(days=1)
+    new = d.start_of("day")
+
+    assert new.day == 7
+    assert_datetime(new, 2025, 9, 7, 1, 0, 0, 0)
+    assert new.offset == -3 * 3600
+    # start_of("day") must be idempotent even across the midnight gap.
+    assert new == new.start_of("day")