fix: carry numeric round/floor/ceil precision >= 60s into minute/hour/day

A numeric $precision passed to round()/floor()/ceil() is always resolved
to the "second" unit. Once the precision reaches or exceeds 60 (e.g.
round(3600) meaning "nearest hour"), the rounding stayed confined to the
second field (range 0-59) and never carried into minute/hour/day, unlike
the equivalent string form (round('hour')).

Re-target the unit to minute/hour/day once the numeric precision reaches
that unit's own span in seconds, rescaling the precision accordingly,
mirroring the existing week -> day re-targeting already in roundUnit().
Sub-minute numeric precisions are unaffected.

tests/Carbon/RoundTest.php

@@ -205,4 +205,36 @@ class RoundTest extends AbstractTestCase
 
         Carbon::now()->roundUnit('foobar');
     }
+
+    public function testRoundWithNumericPrecisionCarry()
+    {
+        // A numeric precision >= 60s must carry, like the equivalent string unit.
+        $dt = Carbon::create(2026, 1, 1, 14, 21, 37);
+
+        $this->assertCarbon($dt->copy()->round(3600), 2026, 1, 1, 14, 0, 0, 0);
+        $this->assertTrue($dt->copy()->round(3600)->eq($dt->copy()->round('hour')));
+
+        $this->assertCarbon($dt->copy()->round(120), 2026, 1, 1, 14, 22, 0, 0);
+        $this->assertTrue($dt->copy()->round(120)->eq($dt->copy()->round('2 minutes')));
+
+        $this->assertCarbon($dt->copy()->round(86400), 2026, 1, 2, 0, 0, 0, 0);
+        $this->assertTrue($dt->copy()->round(86400)->eq($dt->copy()->round('day')));
+
+        // floor() with numeric precision must also carry, not just clamp within the minute.
+        $almostNextHour = Carbon::create(2026, 1, 1, 14, 59, 59);
+        $this->assertCarbon($almostNextHour->copy()->floor(3600), 2026, 1, 1, 14, 0, 0, 0);
+        $this->assertTrue($almostNextHour->copy()->floor(3600)->eq($almostNextHour->copy()->floor('hour')));
+
+        // ceil() with numeric precision must carry across the hour boundary.
+        $halfPastHour = Carbon::create(2026, 1, 1, 14, 30, 0);
+        $this->assertCarbon($halfPastHour->copy()->ceil(3600), 2026, 1, 1, 15, 0, 0, 0);
+        $this->assertTrue($halfPastHour->copy()->ceil(3600)->eq($halfPastHour->copy()->ceil('hour')));
+
+        // Carry across a month boundary via a day-level numeric precision.
+        $monthEnd = Carbon::create(2026, 1, 31, 20, 0, 0);
+        $this->assertCarbon($monthEnd->copy()->round(86400), 2026, 2, 1, 0, 0, 0, 0);
+
+        // Sub-minute numeric precision must keep rounding within the second field, unchanged.
+        $this->assertCarbon($dt->copy()->round(30), 2026, 1, 1, 14, 21, 30, 0);
+    }
 }