Reconcile BSplines with auto_curvature with neighbors

`Paths.reconcilefields!` could reconcile BSplines created with `auto_curvature` so that the endpoint curvature is updated to match its neighbors.

This would require making `auto_curvature` (and `auto_speed`, since we would re-optimize speed under the curvature constraint) stored in the BSpline itself.

This would change the existing documented behavior of `auto_curvature`, so it would be a breaking change.

For reference, there is already reconciliation of curvature in #123, but it is done separately from other reconciliation.

Start at Paths.reconcilefields! and inspect how BSplines store or use auto_curvature and auto_speed. Read the separate curvature reconciliation in issue #123, then determine how reconciliation should update endpoint curvature and re-optimize speed while preserving the intended breaking-change behavior.

Written by the indexing model from the issue text.