Add full-width cut guides between rows

Introduces a distinct "cut guide" concept: a full-width horizontal line below every row, kept 6mm clear of the tokens above (and below, where a further row exists). The row gap is widened to 1.2cm (twice the 6mm margin) to fit this cleanly. Drawn for every row, including a lone top row, so a single row can still be cut away from the sheet.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com