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>
