[RodStraightSection] Fix rest shape double-offset for non-first sections

[lkarstensen]

Summary

Fixes #223.

RodStraightSection::getRestTransformOnX computed the rest position as x_start + x_used, but x_used is already the global curvilinear abscissa. This placed rest-shape nodes at 2 * x_start + local_offset instead of x_used for any section that is not the first in the wireMaterials list.

Example (stiff section 420 mm + flex section 30 mm):

	x_start	x_used	Old result	Correct
Section 0	0	210	210 ✓	210
Section 1	420	430	850 ✗	430

The ~420 mm offset between rest shape and mechanical DOFs caused AdaptiveBeamForceFieldAndMass to generate huge elastic restoring forces every timestep, leading to simulation explosion on any disturbance (e.g. vessel-wall contact).

Fix

// Before
global_H_local.set(type::Vec3(x_start + x_used, 0.0, 0.0), Quat());

// After
global_H_local.set(type::Vec3(x_used, 0.0, 0.0), Quat());

This matches the pattern already used correctly by RodSpireSection and RodMeshSection. Zero effect on single-section wires (x_start = 0).