feat: add sanity checks to `register_sym_simp`

This PR adds validation to the `register_sym_simp` command:
- reject duplicate variant names
- validate `pre`/`post` syntax by elaborating them via `elabSymSimproc`
  in a minimal `GrindTacticM` context, catching unknown theorem names
  and unknown theorem set references at registration time

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2 files changed