Refactor Einsum prover/verifiers: deduplicate `bmk_*` and `mbk_*` variants

[Forpee]

In the Einsum proof system, a few equation variants appear to share essentially the same proving/verifying structure, with only small differences in how operands are indexed/transposed:

bmk_bkn_mbn and bmk_kbn_mbn
mbk_bnk_bmn and mbk_nbk_bmn

Today these are implemented as separate prover/verifier modules and dispatched independently in mod.rs

Proposal

Refactor the prover(s) so each “pair” of structurally-similar einsums shares one implementation:
One shared implementation for the bmk_*bn*->mbn pair
One shared implementation for the `mbk_*bk->bmn pai``