nla-qwen2.5-7b-L20-av — re-warm-started on Sonnet-4.6 "matryoshka" data

• Warm-start data: syvb/nla-qwen2.5-7b-L20-matryoshka-warmstart-sonnet46 (av-* split, document-level; ~5k held-out for eval)
• Hyperparameters (Qwen2.5 recipe): global batch 256, lr 2e-5→2e-6 cosine, warmup 50, 1 epoch, injection_scale 150
• This is a warm-start checkpoint intended as the starting point for a subsequent RL phase.