This adds cxxbridge-cmd to Cargo.lock per https://github.com/dtolnay/cxx/issues/1407#issuecomment-2509136343 It adds an MSRV to `src/taskchampion-cpp/Cargo.toml` so that the version of `Cargo.lock` is stil compatible with the MSRV. It additionally adds a check of the Cargo metadata for all of the cxx* versions agreeing, and for the MSRV's agreeing.