tighten PDF grid for 6×6 layout — CODE_SIZE 80→72, ROW_GAP 18→12

@@ -34,10 +34,13 @@3434 const HEADER_HEIGHT = 44;3535 const HEADER_HAIRLINE = 0.5;3636 37-const CODE_SIZE = 80;37+// 6×6 = 36 codes per page. CODE_SIZE chosen so the 6-row stack clears the38+// footer with ~90pt of breathing room after labels; the column-gap formula39+// adapts automatically to CODES_PER_ROW.40+const CODE_SIZE = 72;3841 const COL_GAP =3942   (PAGE_WIDTH - 2 * MARGIN - CODES_PER_ROW * CODE_SIZE) / (CODES_PER_ROW - 1);40-const ROW_GAP = 18;43+const ROW_GAP = 12;4144 4245 const FOOTER_HEIGHT = 32;4346 const FOOTER_HAIRLINE = 0.5;