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>

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.
 }