DescriptionText.parseTimestamp turned a matched timestamp string into a
clickable seek link by computing seconds from its colon-separated parts
with no range validation. The link regex (\d{1,2}:\d{2}(?::\d{2})?)
matches strings that are not valid clock positions, such as 1:99 or
0:60, and parseTimestamp happily computed 1*60+99 = 159 for 1:99 — so
tapping such a link seeked the player to 2:39 instead of being ignored.
This also diverged from ChapterParser, which rejects seconds/minutes
>= 60; the same timestamp string could be dropped as a chapter but
still seek as a description link.
- Validate seconds < 60 in the MM:SS branch and minutes < 60, seconds
< 60 in the H:MM:SS branch; return nil otherwise
- Change parseTimestamp to return Int? and skip building the seek link
for nil, so out-of-range matches are left as plain text
- Add DescriptionTextTests with valid, boundary, and out-of-range cases
