v0.12.0

• Fix cross-entropy targets, stale caches, attention masks, nonfinite best values,
  and mixed-dtype evaluation.
• Correct simplex gradients and whole-model trust predictions; migrate checkpoints
  to format 5.
• Reuse MLP prefixes, use native CPU attention, and compile site-local evaluation.
• Average tied baseline ranks; add controlled experiments and forward benchmarks.
• Validate sparse dimensions and factored ranks; simplify documentation.