fix(subtitles): improve secondary subtitle extraction and display (#215)

changes/dedupe-embedded-subtitle-extraction.md

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+type: fixed
+area: subtitles
+
+- Prevented embedded subtitle parsing from starving network playback: mounted SMB/NFS media now uses deduplicated mpv live text, while duplicate extraction requests for local media share one ffmpeg process.

changes/remove-secondary-subtitle-display-limit.md

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+type: fixed
+area: overlay
+
+- Secondary subtitle overlays now show every rendered line instead of clipping text after roughly four lines.