This PR prevents `Sym.simp` from looping on permutation theorems like
`∀ x y, x + y = y + x`.
- Add `perm : Bool` field to `Theorem`
- Add `isPerm` that checks if LHS and RHS have the same structure with
pattern variables (de Bruijn indices) rearranged via a consistent
bijection. Uses `ReaderT` (offset for binder entry), `StateT`
(forward/backward maps), `ExceptT` (failure).
- Compute `perm` in `mkTheoremFromDecl` / `mkTheoremFromExpr`
- In `Theorem.rewrite`, when `perm` is true, only apply the rewrite if
the result is strictly less than the input (using `acLt`)
- Tests include the classic AC normalization stress test with
`add_comm`, `add_assoc`, `add_left_comm`
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
