Add a focused regression that `compute_stress` lands on the von Mises surface when
the trial state lies outside yield, using the returned backstress state.
- SPDX header refresh for the file.
New 229-line test file included in main test/runtests.jl:
- Tests PerfectPlasticity material instantiation and traits
- Tests global material cache creation for stateful materials
- Tests direct NamedTuple state manipulation (compositional design)
- Tests cache access and manipulation (set_state!, update_cache!)
- Tests multi-step plasticity simulation workflow
- Tests state variable helpers (get_state_variable, set_state_variable)
- Validates type stability of key operations
Comprehensive integration test ensuring plasticity material works with cache system.
Jukka Aho