fix: start animated AVIF motion at sentence start, not audio padding

- Removed leading padding from AVIF clip start so pre-sentence frames are not shown
- Duration now extends by trailing padding only
- Added regression test verifying -ss and -t args against a stub ffmpeg

changes/animated-avif-padding-freeze.md

@@ -0,0 +1,4 @@
+type: fixed
+area: anki
+
+- Kept animated AVIF card images from showing pre-sentence motion on multi-line mines without delaying motion after sentence audio starts.