[.ai] generalize single-file model layout section
Drop the implementation-specific examples (Lumina class names, per-model
transformer_*.py citations) and state the single-file policy generally:
everything (attention, blocks, RoPE, model-specific layers) in one model
file, with shared blocks either imported from the common modules or brought
in via `# Copied from ... with Old->New`.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
