[visual-qa] EditVertex drag on clothoidRailBend translates the clothoid instead of scaling L

[grootstebozewolf]

Part of #68. Follow-on to #88 (type kept).

Observed

clothoidRailBend COMPOUNDCURVE: dragging a clothoid end (e.g. 116095.653 412104.165) keeps L and translates the whole clothoid so the chord length is unchanged. The spiral looks bent / jumps off the preceding member.

Cause: GeometryEditor.rebuildClothoid, when only the end moves, shifts start by the same delta and passes the old L to createClothoid.

Expected

After a vertex drag, rebuild from the new start, keep kappa and start tangent, and scale L by newChord/oldChord (the edited control chord). Do not translate start when only the end is dragged.

Not

• Not flatten-to-LINESTRING ([visual-qa] EditVertex on clothoidRailBend COMPOUNDCURVE flattens to LINESTRING #88).
• Not extract CLOTHOID honesty ([visual-qa] Extract elements from clothoidRailBend drops CLOTHOID to a 2-point LINESTRING #95).

[grootstebozewolf]

PR #102: rebuildClothoid scales L by newChord/oldChord and keeps start (no translate). Do not close without UX SIGN.