feat: add unconditional diffusion model path

[aegioscy]

Summary

• Add sd_ctx_params_t::uncond_diffusion_model_path for loading a standalone unconditional diffusion model.
• Load the unconditional weights under the model.diffusion_model.uncond. prefix, matching Ideogram 4 CFG GGUF layout.
• Expose the path through --uncond-diffusion-model, context logging, and generated metadata.

Test plan

• cmake -S . -B build-uncond -DSD_BUILD_EXAMPLES=OFF -DSD_BUILD_SHARED_LIBS=OFF -DGGML_NATIVE=OFF
• cmake --build build-uncond --config Release -j 8

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