feat: add extensible state mechanism for SymM

This PR add `SymExtension`, a typed extensible state mechanism for `SymM`,
following the same pattern as `Grind.SolverExtension`. Extensions are
registered at initialization time via `registerSymExtension` and provide
typed `getState`/`modifyState` accessors. Extension state persists across
`simp` invocations within a `sym =>` block and is re-initialized on each
`SymM.run`.

This enables modules (e.g., the upcoming arithmetic normalizer) to
register persistent state without modifying `Sym.State` directly.

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