Periodic parallel-velocity spline fix moved to PR #341 #340

The reviewed implementation is now available as draft PR #341.

It uses a periodic cubic for the closed-poloidal-turn parallel-velocity factor, enumerates bounce roots exactly within cubic spans, and adds an independent analytic seam/root regression. The PR contains the concise rationale and validation summary.

Closing this temporary handoff issue in favor of the actual pull request.