Phase 2: Wire up PPX preprocessing in execute
The JsooTopPpx module was set up to capture PPX registrations and apply
rewriters, but the preprocess_phrase function was never called during
execution. This commit:
- Adds JsooTopPpx.preprocess_phrase call before Toploop.execute_phrase
- Removes debug output from preprocess_structure
This enables PPX rewriters (including Ppx_js for [%js] syntax) to work
properly in the toplevel.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>