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F6 §6.8. Drives the programmatic seam ingress (propose + accept, no LLM): opened baseline → toggle opens cowriting-baseline diff → type (baseline steady) → accept advances past the landing (INV-18) → operator delta → pin → on-disk baseline outside .threads (INV-19) → toggle closes (normal editor active) → untracked warns. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>