Pulls the rapid-walk logic (sub-program unwrapping, first-pierce lookup, incremental-vs-absolute handling, first-rapid skipping) out of PlateRenderer.DrawRapids into a reusable RapidEnumerator in Core so it can be unit-tested and reused outside the renderer. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>