This PR fixes a kernel type mismatch error in grind's denominator
cleanup feature. When generating proofs involving inverse numerals (like
`2⁻¹`), the proof context is compacted to only include variables
actually used. This involves renaming variable indices - e.g., if
original indices were `{0: r, 1: 2⁻¹}` and only `2⁻¹` is used, it gets
renamed to index 0.
The bug was that polynomials were correctly renamed via `varRename`, but
the variable index `x` stored in `cancelDen` constraints was passed
directly to the proof without renaming, causing a mismatch between the
polynomial's variable references and the theorem's variable argument.
Added `ringVarDecls` to track ring variable indices that need renaming,
similar to how `ringPolyDecls` tracks polynomials. The `mkRingContext`
function now also renames these variable indices.
See zulip discussion at [#nightly-testing > Mathlib status updates @
💬](https://leanprover.zulipchat.com/#narrow/channel/428973-nightly-testing/topic/Mathlib.20status.20updates/near/560575295).
🤖 Prepared with Claude Code
Co-authored-by: Claude <noreply@anthropic.com>
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