Add a just update-outfiles helper script

[dlqqq]

Motivation

Regenerating a version's .env.out lockfiles today means running the full src/main.py build, which builds the entire image (CPU and GPU) just to end up calling micromamba env export --explicit on the base env. That is far more than is needed to refresh a lock after editing an .env.in (e.g. bumping a dependency or pinning a channel), and it isn't discoverable.

Summary

Adds a top-level justfile and adds just to the dev environment.yml. A lockfile is just the fully-resolved base conda environment, so the recipe reproduces exactly that: it solves the {cpu,gpu}.env.in (plus pinned_env.in, and the CUDA arg_based_env.in for GPU) in the matching mambaorg/micromamba base image and records it with micromamba env export --explicit. No full image build, and no GPU hardware — GPU packages resolve against the CONDA_OVERRIDE_CUDA virtual package.

Recipes:

• just update-cpu-outfile <version>
• just update-gpu-outfile <version>
• just update-outfiles <version> (both)

You can verify it against an unchanged .env.in: regenerating should leave the committed .env.out unchanged apart from packages that legitimately have newer builds on conda-forge.