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 {
-		return t, nil, unstable.NewParserError(b[6:8], "seconds cannot be greater 60")
+	if t.Second > 59 {
+		return t, nil, unstable.NewParserError(b[6:8], "seconds cannot be greater than 59")
 	}
 
 	b = b[8:]