Salpa 0.2.1

Three fixes since 0.2.0.

Apple Silicon now installs packages that ship x86_64 builds. Packages such as
metaldock-vwf depend on tools with no arm64 build anywhere (AutoDock 4, MGLTools).
Salpa used to refuse these on Apple Silicon outright. It now installs them, and macOS
runs them through Rosetta. If Rosetta is missing, Salpa says so and gives you the exact
command:

softwareupdate --install-rosetta --agree-to-license

That check matters more than it looks: pixi does not verify Rosetta before falling back,
so without it a Mac lacking Rosetta would report a successful install whose every
binary then fails to launch. Intel Macs and Linux are unchanged.

A stale environment is no longer trusted because it exists. After an update, Salpa
verifies that its Python environment resolves inside the running build, and rebuilds it
if not, instead of silently running an older version's code.

Copilot installs packages the same way every other path does, through the shelf.