Implement object distribution p_obj(x) (pink noise sampler)

[raeesiarya]

Replace file-based loading of ground-truth images with a generative process that samples x from a known distribution p_obj.
Phase 1: pink noise.

Acceptance Criteria
• Add sample_pink_noise_batch(batch_size, size) (returns tensor of shape (B,1,H,W))
• Add config key: obj_type = "pink"
• Modify testbench so that if obj_type == "pink":
• skip loading files
• directly sample synthetic x_true