Runtime commands for a derive-generated usage-rs host.
A CLI that has plugins knows what host plugin-x is only after it has read its own configuration,
which is long after the tables describing the rest of its CLI were compiled. Those tables
stay as they are: an application discovers and caches plugin specs itself, and a [Catalog]
attaches them beneath a static command that declared an
external_subcommand catch-all.
What that buys is the three things the static tables cannot answer for a command they have never seen: help that lists and navigates runtime commands, completion that descends into them, and a parse of the argv the catch-all captured. Nothing here mutates the derived parse tables or the KDL they emit, and nothing here runs a subprocess, reads a file, or calls back into the application.