chore: re-add cbv at, but now adhering to SymM invariants

[wkrozowski]

This PR adds cbv at feature to run cbv on local hypotheses, but now it is safe with respect to SymM invariants, namely, each cbv call (to a local hypothesis) is contained within a single SymM context, which remains incremental

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d19a5e526c29315b07683839666a8aa95d9f7e69 --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-30 13:43:56)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase d19a5e526c29315b07683839666a8aa95d9f7e69 --onto a39eab69e1eee9ad38f4efe507907b1026a77808. You can force reference manual CI using the force-manual-ci label. (2026-07-30 13:43:57)