Takeaway: Next time I'd tell it upfront that melt and xmllint passing doesn't mean kdenlive itself will open the file cleanly — actually launching the real app was the check that mattered, not the automated gate.
Takeaway: Next time I'd tell the agent upfront that only the actual Kdenlive GUI validates file hashes and track playlist structure — melt and xmllint gave false confidence twice and I only caught the real bugs by opening the app myself and eyeballing the dialog.
Takeaway: Next time I'd tell it upfront that source footage is mixed-resolution (not all 4K) so the vertical bake gets written resolution-independent from the start instead of two rounds of fixes.