Add double bed jacquard punchcard generation

[Copilot]

The generator only supported standard 1:1 punchcards. This adds double bed jacquard support, where each design row is split into two punchcard rows: the original pattern row (colour selection) plus a configurable backing row.

New --jacquard flag

Mode	Backing row
none	Standard 1:1 card (default, no change to existing behaviour)
doublebed	All stitches punched — solid/full-jacquard backing
birdseye	Alternating checkerboard (x-x-x-...), phase-shifted each pair for a diagonal bird's eye grid
tuck	Inverse of the pattern row — tuck-stitch backing

New function

build_double_bed_rows(rows, backing) applies the transformation after compose_card_rows, so layout and --repeat-height logic runs first and then rows are doubled.

Usage

python3 punchcard-generator.py design.png 24 motif 6 --jacquard doublebed
python3 punchcard-generator.py design.png 24 motif 6 --jacquard birdseye
python3 punchcard-generator.py design.png 24 motif 6 --jacquard tuck

A 6-row motif with --jacquard doublebed produces a 12-row card.