Register the recently introduced damage/creep/eigenstrain/Chaboche markers so the
generated registry cannot drift from their storage types and default symbols.
- SPDX header refresh.
New 93-line test file included in main test/runtests.jl:
- Tests AbstractStateVariable type hierarchy
- Validates state_variable_type trait (returns concrete types)
- Validates default_symbol trait (returns symbols like :ε_p, :α, :κ, :d)
- Tests concrete type instantiation (PlasticStrain, Backstress, etc.)
- Tests NamedTuple usage with state variable symbols
- Verifies trait consistency across all state variable types
Ensures state variable system works correctly for material models.
Jukka Aho