By: ValentinHirschi <valentin.hirschi@gmail.com>
preserve completed contractions across tensor sums Merge occurrence counts across alternative reconstruction branches while retaining open-index reservations and incidence validation. Cover nested closed sums, squared sums, shared external indices and factorized exact canonicalization. Based on gammaloop's existing 0441bd7 dependency revision. Validation: cargo check --lib --tests; nine tensor canonicalization tests pass. User request: OK, and make sure the fix to `Idenso reports a Lorentz dummy contracted more than once` is generic. We really need a robust and generic gammaloop.