propagate stl alignment to ttml

[dlecorfec]

The left/right alignment of STL subs was lost when converting to TTML.

[asticode]

Thanks for the PR! 👍

Could you add some tiny tests for that?

[dlecorfec]

(AI-generated tests by Claude Sonnet 4.5 with prompt "add tests for commit d52237e", it generated a bit more than asked, but it seems fine)

Added TestPropagateSTLAttributes to subtitles_internal_test.go with 9 test cases:

1. JustificationRight - Verifies that JustificationRight propagates to both WebVTTAlign and TTMLTextAlign with value "right"
2. JustificationLeft - Verifies that JustificationLeft propagates to both WebVTTAlign and TTMLTextAlign with value "left"
3. JustificationCentered - Verifies that JustificationCentered doesn't set TTMLTextAlign (centered is default behavior)
4. NoJustification - Verifies that when justification is nil, no alignment attributes are set
5. STLPositionInVision - Tests vertical position conversion for in-vision subtitles (up to 99 rows)
6. STLPositionTeletext - Tests the special teletext conversion that subtracts 1 from position for better rendering
7. STLPositionTeletextRow0 - Edge case where position 0 doesn't subtract 1
8. JustificationAndPosition - Tests that both justification and position features work correctly together

[asticode]

Thanks for this PR too ❤️

Let me know whether you need a tag 👍

[dlecorfec]

Thanks, I think a tag is in order to celebrate the day! 🚀

[asticode]

FYI I've created a v0.39.0 tag 👍