[M0-S4] export_atlas.py: sprite board → Phaser atlas #61

Problem

Sprite boards are reviewable images, not game assets. The game needs frames sliced, scaled and named.

Acceptance criteria

• python scripts/export_atlas.py <board> slices frames by rows and alpha, scales to game size (hero 80 px tall, feet on one baseline row) and writes game/public/assets/<board>.png + .json (Phaser atlas)
• Frame names <animation>-<nn>; animation names and frame counts come from the board config, matching docs/07
• Fails loudly on a frame count mismatch
• Unit tests in scripts/test_export_atlas.py

Notes and dependencies

Board format: 09 – Sprite boards. Frame lists: 07 – Asset manifest. Suggested owner: Alexander (pipeline, art, scenes, deploy).

Verification

Exports the approved hero sheet; the atlas opens in the game and in a Phaser atlas viewer.