Update model card: prism_medium/expressive_medium checkpoints + training status

Remove superseded expressive_medium-step56000.safetensors

Remove superseded prism_medium-step58000.safetensors

expressive_medium: step 56000 -> 90000 (training in progress)

prism_medium: step 58000 -> 376000 (training in progress)

Remove legacy yellow.pt, superseded by yellow_small-final.safetensors

Add yellow_small checkpoint (final)

Add yellow_medium checkpoint (final)

Add expressive_medium checkpoint (step 56000, training in progress)

Add prism_medium checkpoint (step 58000, training in progress)

Update README.md

Add correct citation

Remove invented citation

Fix model card: correct API example, remove broken image

Add yellow model weights

Add yellow encoder config

Add model card

initial commit