Tag the ltxv and wan GGUF archs text-to-video so cached video checkpoints
actually surface in the Video picker (they were classed unsupported and
hidden everywhere). Adopt the loaded family's default clip length instead
of silently keeping the 25-frame pre-load fallback, and derive steps and
guidance from the picked GGUF filename so a distilled variant gets its
few-step schedule. Suppress the error toast for the user's own Cancel and
disable the Video nav item on chat-only hosts with a hint, matching Train.
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