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This PR optimizes the `String.reduceEq`, `String.reduceNe`, and `Sym.Simp` string equality simprocs to produce kernel-efficient proofs. Previously, these used `String.decEq` which forced the kernel to run UTF-8 encoding/decoding and byte array comparison, causing 86+ kernel unfoldings on short strings. The new approach uses `String.ofList_injective` combined with `congrArg (List.get?Internal · i)` at the first differing character position, reducing kernel work to O(first_diff_pos) list indexing plus a single character comparison. For equal strings, `eq_true rfl` avoids kernel evaluation entirely. The shared proof construction is in `Lean.Meta.mkStringLitNeProof`, used by both the standard simprocs and the `Sym.Simp` ground evaluator. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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