Turns out the solution to my broken Staticmatic install was as simple as:
bundle install
Yeah. That simple. Durr. But big respect for Bundler, a simple tool that solves a subtle problem – in this case downgrading the versions of a bunch of gems, rolling them back to whatever I built this system with in the first place, but doing all that only within this project.
Yeah, I know that’s what it does, but I didn’t really know that’s what it does. I should probably RTFM for some of this stuff.