Overhaul `infra` script

The `infra` script is now a Frankenstein monster of way too many features tacked on to a basic frame that was never intended to get so complicated. It would be great to have fewer fragile mechanisms for configuration passing via the command line.

It's probably worth looking into the [`getopts`/`getopt`](https://www.baeldung.com/linux/bash-parse-command-line-arguments) mechanism for properly processing options as a starting point.

Start by reading the `infra` script and tracing how it currently passes configuration through command-line arguments. The issue suggests evaluating Bash `getopts`/`getopt`, but it does not define a concrete option set, affected files beyond `infra`, or tests. Done would mean a less fragile configuration interface, with its existing behavior preserved and verified.

Written by the indexing model from the issue text.