Shorthand way to benchmark against `main`

## Is your feature request related to a problem? Please describe

Following from #1442, it would be nice to have an easy way to benchmark performance against the `main` branch. Currently this is possible by switching to the `main` branch, running the benchmarks, then switching to the feature branch, running benchmarks again and opening the report. This is ok, but it would be nice to have a single command to do all this for you.

## Describe the solution you'd like

A simple command that lets us benchmark the current branch against `main` (possibly a `just` command)

## Describe alternatives you've considered

Do nothing - the current approach works but is a bit annoying

## Additional context

Could run this as part of CI on pull requests

I wonder if there's some overlap with the approach we've used for building docs with different versions?

Start by reviewing the workflow described in #1442 and the repository's existing benchmark commands or `just` recipes. Compare the current `main` and feature-branch benchmark steps, then determine whether the single command should also support pull-request CI; done means one reproducible command produces the comparison report.

Written by the indexing model from the issue text.