Hygiene: compiler warnings, missing [[nodiscard]], dead members

[petlenz]

The tree is clean under ASan + UBSan; these are the leftovers.

• -Wreorder in materials/linear_elasticity.h
• -Wshadow in core/material_base.h:29-31
• unused typedef at io/json_material_factory.h:26
• [[nodiscard]] on local_newton::solve and vector_newton::converged -- a dropped convergence flag is the bug this codebase has had most often
• material_base::m_material_handler (core/material_base.h:54) is stored and never read
• statev_map holds three std::function per entry; a small variant would do

Once clean, add -Wshadow -Wreorder to the CI warning set so they stay that way.

All references are to main at 91bd26f.