Update test_cache_updates.jl to support both legacy and new
GlobalMaterialCache API with proper allocation testing.
- Rename legacy test set to indicate it uses old API
- Update field access to use get_stress/get_tangent getter functions
- Relax allocation test for legacy API (may have NamedTuple overhead)
- Add new test set for GlobalMaterialCache API
- Add zero-allocation verification using BenchmarkTools
- Test both material_cache.σ/𝔻 access patterns