194-the-ledger-probe-sweeps-the-candidate

The built-in ledger probe sweeps the candidate, not origin/main

Spec 154's carried acceptance proves the built-in exempt ledger by sweeping spec 019 at `origin/main` with the candidate binary. Under an exact engine pin, a version bump makes the candidate refuse `origin/main`'s older pin, so 046, 089, 146 and 154 fail in the merge queue of every bump. The probe sweeps the candidate's own `HEAD` instead.