Editor v2 backend: an edit_spec can carry a 'segments' cut-list that concatenates the kept
ranges into ONE file (accurate=filter_complex trim+concat re-encode; fast=concat-demuxer
stream-copy with per-segment inpoint/outpoint). normalize/edit_sig/clip_duration/needs_reencode
handle segments; worker writes the concat list + runs build_concat_plan. Single-trim path (used by
'separate files' export) unchanged.