refactor: add scoping backend seam for JuliaLowering spike

## Summary - extract resumable scoping behind a backend seam - preserve current `legacy` scoping behavior by default - add an explicit experimental hook for a future JuliaLowering-backed path - add backend-selection tests around the new seam ## Why Issue #99 is now primarily about the JuliaLowering-based direction rather than re-fixing the already-landed ad-hoc scoping work. This PR does **not** claim to complete JuliaLowering integration. Instead, it ships the first safe preparatory slice: - removes the hardcoded scoping pass from the macro body, - creates a narrow swap point for experimentation, - keeps package behavior unchanged by default, - makes future JuliaLowering work incremental and reviewable. ## Current behavior - `RESUMABLEFUNCTIONS_SCOPE_BACKEND=legacy|julialowering` - default remains `legacy` - unsupported values fail early with a clear error - `julialowering` is currently an explicit stub/not-yet-implemented hook ## Validation - `julia --project=. -e 'using Pkg; Pkg.test(; test_args=["main"])'`