Cleaning everything on app.toml changes shouldn't really be necessary

Currently, xtask dist runs around cleaning things whenever app.toml changes. This was originally done so that switching between apps didn't accidentally link in each others' objects. It's kind of annoying, however, and shouldn't be strictly necessary ... if we've got all the various build dependencies right. It may be masking cases where we forgot to (say) emit a cargo:rebuild-if-whatever line from the build script.

It would be great for one of us to investigate and make this more precise, as it should dramatically reduce build times for people working on more than one app.

It's also going to become increasingly important as we move more config information into the toml, since right now, changing (say) the proposed i2c config tree will force a rebuild of all tasks, which is overkill.

Start by tracing how xtask dist reacts to changes in app.toml, then inspect the build scripts and their cargo:rebuild-if-whatever dependencies. Identify which configuration changes actually require rebuilding or cleaning instead of triggering a full cleanup. Done means switching apps and changing targeted TOML configuration no longer causes unnecessary rebuilds while avoiding stale objects.