test: pin the first-flip carry boundary (decimal/traditional/binary)

Per review on #600: the rollover tests only pinned the far end
(999_999 / 1024**2 - 1), not the first value whose mantissa rounds up
to base. Add the carry boundary for all three formatters so the fix's
behavior at the rollover point is locked down.

Author: patchwright

tests/test_filesize.py

@@ -46,17 +46,26 @@ def test_decimal(self):
         self.assertEqual(filesize.decimal(1200 * 1000), "1.2 MB")
 
     def test_rollover_decimal(self):
+        # First value whose mantissa rounds up to base — the carry boundary
+        self.assertEqual(filesize.decimal(999_949), "999.9 kB")
+        self.assertEqual(filesize.decimal(999_950), "1.0 MB")
         # Mantissa rounds up to base — must carry to the next suffix
         self.assertEqual(filesize.decimal(999_999), "1.0 MB")
         self.assertEqual(filesize.decimal(999_999_999), "1.0 GB")
         self.assertEqual(filesize.decimal(999_999_999_999), "1.0 TB")
 
     def test_rollover_traditional(self):
+        # First-flip carry boundary: 1,023.95 KB rounds up to 1,024.0 KB -> 1.0 MB
+        self.assertEqual(filesize.traditional(1_048_524), "1,023.9 KB")
+        self.assertEqual(filesize.traditional(1_048_525), "1.0 MB")
         # 1024**2 - 1 bytes is 1023.999 KB, which rounds to 1,024.0 KB
         self.assertEqual(filesize.traditional(1024**2 - 1), "1.0 MB")
         self.assertEqual(filesize.traditional(1024**3 - 1), "1.0 GB")
 
     def test_rollover_binary(self):
+        # First-flip carry boundary: 1,023.95 KiB rounds up to 1,024.0 KiB -> 1.0 MiB
+        self.assertEqual(filesize.binary(1_048_524), "1,023.9 KiB")
+        self.assertEqual(filesize.binary(1_048_525), "1.0 MiB")
         self.assertEqual(filesize.binary(1024**2 - 1), "1.0 MiB")
         self.assertEqual(filesize.binary(1024**3 - 1), "1.0 GiB")