feat: add `register_sym_simp` command and `SymSimpVariant` infrastructure

This PR adds the `register_sym_simp` command for declaring named `Sym.simp`
variants with `pre`/`post` simproc chains and optional config overrides.

- `SymSimpVariant` structure storing `pre`/`post` syntax (elaborated at use
  time) and `Config` overrides
- `SimpleScopedEnvExtension` for persistent cross-module variant storage
- `register_sym_simp` command syntax with `sym_simp_field` category
- Command elaborator parsing fields and registering variants
- `getSymSimpVariant?` lookup function
- `deriving Inhabited` on `Simp.Config` for extension support

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