tier5 route [2^32,2^64) to base-16 modmul decoder (htop90=5)

tier-4: update weights.pt (htop90=4)

tier-4: update model.py (htop90=4)

Upload modmul_t4_v2_best.pt with huggingface_hub

Add model card

tier-3 modmul scratchpad: htop90=3 (tier1 1.0, tier2 1.0, tier3 0.99)

initial commit