The generated config failed the real loader (KeyError: cuda). It now copies
src/CelerisLab/configs/config_lbm.json and overrides only grid, physics,
collision/streaming/storage, y walls, LES off and uniform inlet profile;
inlet scheme and outlet mode stay the template's and are recorded in
case.json. New CPU test loads the generated file with load_lbm_config.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>