`launch.py` import times are slow

@MisteFr reports slow launch times on the ETH cluster when running `uv run scripts/launch.py shards ...`. I suspect this is a problem because it needs to import every subcommand's package.

There are some issues with directly running modules in `src/saev/framework` because of submitit + cloudpickle + jaxtyping (see [this open issue](https://github.com/patrick-kidger/jaxtyping/issues/332)) but perhaps there is a better way to do this.

Start with scripts/launch.py and profile `uv run scripts/launch.py shards ...` to confirm whether importing every subcommand causes the delay. Read the related code under src/saev/framework and account for the submitit, cloudpickle, and jaxtyping constraints. Done means launch imports are faster without breaking command execution or serialization.

Written by the indexing model from the issue text.