Reject leap seconds to prevent year overflow (#1019)

Go's time.Date() normalizes leap seconds (second=60) by adding 1 minute.
When parsing the maximum valid TOML date 9999-12-31 23:59:60z, this causes
the year to overflow to 10000, which exceeds the valid TOML year range
(0000-9999) and breaks round-trip serialization.

The fix rejects leap seconds (second > 59) during parsing. This is
consistent with the resolution of issue #913 which determined that
emitting an error is less surprising than silently normalizing leap
seconds.

Fixes #1015

Co-authored-by: Claude <noreply@anthropic.com>

decode.go

@@ -230,8 +230,8 @@ func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 		return t, nil, err
 	}
 
-	if t.Second > 60 {
+	if t.Second > 59 {
-		return t, nil, unstable.NewParserError(b[6:8], "seconds cannot be greater 60")
+		return t, nil, unstable.NewParserError(b[6:8], "seconds cannot be greater than 59")
 	}
 
 	b = b[8:]

localtime.go

@@ -45,7 +45,7 @@ func (d *LocalDate) UnmarshalText(b []byte) error {
 type LocalTime struct {
 	Hour       int // Hour of the day: [0; 24[
 	Minute     int // Minute of the hour: [0; 60[
-	Second     int // Second of the minute: [0; 60[
+	Second     int // Second of the minute: [0; 59]
 	Nanosecond int // Nanoseconds within the second:  [0, 1000000000[
 	Precision  int // Number of digits to display for Nanosecond.
 }

unmarshaler_test.go

@@ -3160,7 +3160,17 @@ world'`,
 		{
 			desc: "invalid seconds value",
 			data: `a=1979-05-27T12:45:99`,
-			msg:  `seconds cannot be greater 60`,
+			msg:  `seconds cannot be greater than 59`,
+		},
+		{
+			desc: "leap second not supported",
+			data: `a=1979-05-27T12:45:60`,
+			msg:  `seconds cannot be greater than 59`,
+		},
+		{
+			desc: "leap second with max date causes overflow",
+			data: `s=9999-12-31 23:59:60z`,
+			msg:  `seconds cannot be greater than 59`,
 		},
 		{
 			desc: `binary with invalid digit`,