feature/rk-integrators: Runge-Kutta multi-stage return mapping

[petlenz]

Branch: feature/rk-integrators

What it implements

• Butcher tableau data structure with factory functions (forward Euler, RK4, implicit Euler, SDIRK3, etc.)
• rk_integrator: unified scalar ODE integrator handling explicit/DIRK/fully-implicit schemes
• rk_plasticity: multi-stage return mapping with configurable tableau
• plasticity_utils.h: shared evaluate_at_state() and compute_trial() free functions
• Autocatalytic reaction example updated for RK integration

Files changed

• 23 files changed, +1827/-28 (largest branch)
• New: rk_integrator.h, rk_plasticity.h, butcher_tableau.h, plasticity_utils.h
• New: test_rk_integrator.cpp
• Updated: curing rate material for RK compatibility

Remaining work

• Merge feature/j2-plasticity and feature/review-fixes first (dependencies)
• Review and merge into main
• Factory integration for JSON-driven tableau selection (documented in TODO_json_converter.md)
• Consider adding higher-order convergence test (SDIRK3 vs implicit Euler accuracy comparison)

Notes

• TODO_json_converter.md documents open work on factory integration (separate task)
• Uses Eigen for fully implicit RK Newton system
• Pre-allocated stage storage to avoid per-step allocation