* fix: bound HDR and video extraction resources
* fix: trim negative video extraction preroll
* fix: skip invisible video extraction windows
* fix: preserve negative-start loop and held tails
* fix: cap finite video slots to source duration
* fix: bound held-tail frame extraction
* fix: plan from playable video duration
* fix: preserve open-ended held video tails
* fix: resolve held tails from decoded frames
* fix: normalize final-frame probe timestamps
* fix: handle unseekable final-frame sources
* fix: dedupe final-frame probes per render
* refactor: clarify output dynamic range contract