fix(gallery/ltx-2.3): add diffusion_model flag to all variants (#9986)

LTX-2.3 entries (dev / distilled, UD-Q4_K_M / Q4_K_M / Q8_0) were
missing the `diffusion_model` option in their overrides. Without it,
gosd.cpp routes the main GGUF through the regular `model_path` code
path in sd.cpp, which doesn't apply the `model.diffusion_model.` tensor
prefix. sd.cpp's LTX-2.3 architecture detection (`VERSION_LTXAV`) in
get_sd_version checks for prefixed tensor names — without the prefix,
detection fails and load_model returns "could not load model".

This is the same bug we hit for Wan when the option was missing.
Adding `- diffusion_model` to all six LTX-2.3 entries' option blocks
makes load_model take the diffusion_model_path branch so detection
succeeds.

Assisted-by: Claude:claude-opus-4-7