docs: link executive briefing PDF in model card

docs: improved model card with training narrative and polaris-revival link

docs: correct training curriculum (6-phase 1500-step sprint), wall-clock ~7 days, Raccoon 6.1B

fix: consistent 2.7B naming, reviewer fixes, rtaforge-substrates note

full model card rewrite — Rabbit v0.1 Alpha

Upload README.md with huggingface_hub

docs: reframe as PoC, add step-1500 internal evals, honest lm-eval disclaimer

model card: CC-BY-NC-SA 4.0, lineage, dataset curriculum, base/imprint structure