fix: strip nested mdata in grind preprocessing

This PR fixes an issue where `grind` would fail after multiple `norm_cast`
calls with the error "unexpected metadata found during internalization".

The `norm_cast` tactic adds mdata nodes to expressions, and when called
multiple times it creates nested mdata. The `eraseIrrelevantMData`
preprocessing function was using `.continue e` when stripping mdata,
which causes `Core.transform` to reconstruct the mdata node around the
visited children. By changing to `.visit e`, the inner expression is
passed back to `pre` for another round of processing, allowing all
nested mdata layers to be stripped.

Closes #11411

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

Co-Authored-By: Claude <noreply@anthropic.com>