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ltx2-gen exposes --first-frame but never the last_frame keyframe the ABI and the engine both serve #1191

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@localai-bot

vllm_video_params carries last_frame (include/vllm.h:948) and the LTX-2.5
engine serves it: wants_last_frame places the image as a
VideoConditionByKeyframeIndex at pixel frame frames - 1
(src/vllm/multimodal/ltx2_video.cpp, the last-frame arm of the phase loop),
gated by test_ltx2_video under row LTX25-TOKEN-APPEND
(#930).

examples/ltx2_gen/main.cpp parses --first-frame (:333) and assigns
vp.first_frame (:413) and never reads last_frame at allgrep -c last_frame examples/ltx2_gen/main.cpp returns 0 for the field. So the closing
keyframe is reachable from the C API and not from the shipped CLI.

This became load-bearing with row LTX25-KEYFRAME-INTERP
(#1096):
KeyframeInterpolationPipeline exists to generate the motion BETWEEN pinned
keyframes, and with one slot the CLI can only ask for half of that. Fixed in
flow by that row, because it is the first caller the gap actually narrows: one
--last-frame flag, parsed and assigned beside --first-frame, sharing the
same --image-crf and noise_aug the two slots already share.

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