_load_installable_optional_extras() was returning ALL extras from
pyproject.toml except 'all', which included 'rl' and 'yc-bench' —
extras not referenced by [all] that install heavy research deps
(atroposlib, tinker, wandb) from git repos. Changed to parse the
[all] group's references and only retry those 18 extras.
Also moved tomllib import to function-level since it only runs
during the rare fallback path.