Add 3 new evidence files from modern open-source sources:
- karpathy_recipe_training_nn_2019.md: Karpathy's training recipe blog post
- nanochat_deepwiki_llm_pretraining_2026.md: 320+ HP sweeps for GPT-2-scale pretraining
- sanh_simple_considerations_hf_2021.md: HuggingFace NLP debugging notes
Add update-to-data ratio diagnostic to refs/diagnostics.md (target ~1e-3).
Add LLM pretraining gap note to SKILL.md intro linking the new sources.
Add tanh saturation % to logging checklist.
Moved heat-exchanger-specific content from pinn/SKILL.md to
pinn/refs/heat_exchanger.md: complexity ladder table, known failure
modes (U->0, counterflow signs), property mappings (REFPROP/PCHIP),
multi-episode training. PINN skill is now domain-agnostic.
pinn/SKILL.md reduced from 4961w to 4274w (~14%).
wassname