Card and audio-less-clip anullsrc sources are infinite; concat never
terminates an infinite segment, so the encode hung forever at 30%. atrim
pins each bed to its item's span (card duration / clip trim).
Per the adopted qwen38 plan §2 (+ glm53 amendments): video_processing_jobs
kind 'assemble' (claim widened via 044), EdlPlan mirror validation
(lenient-fail), the filter_complex builder (interleaved concat pairs, fit
cover/contain chains, absolute-timeline PNG overlays with enable windows,
VO adelay/sidechain duck, hard-breaking word wrap for card PNGs), and the
runner's assemble path with pre-spawn trim gates, attempt backoff and 15-min
cap. Sources open by magic (SPENC1 kek / SPENC2 direct key) so direct-to-R2
uploads assemble without the api ever seeing bytes. Tests: builder 18,
edl-plan 12, overlay-png 3, runner fakes extended.
