fix: mark unused variable with underscore

The `lastCharWasDigit` parameter in the fold lambda was not used in the
function body, only tracked in the result tuple. Mark it as unused with
an underscore prefix to satisfy the linter.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: kim-em

src/Init/Data/String/Slice.lean

@@ -1277,7 +1277,7 @@ def isNat (s : Slice) : Bool :=
     false
   else
     -- Track: isFirst, lastWasUnderscore, lastCharWasDigit, valid
-    let result := s.foldl (fun (isFirst, lastWasUnderscore, lastCharWasDigit, valid) c =>
+    let result := s.foldl (fun (isFirst, lastWasUnderscore, _lastCharWasDigit, valid) c =>
       let isDigit := c.isDigit
       let isUnderscore := c = '_'
       let newValid := valid && (isDigit || isUnderscore) &&