Create src/trusses/api.jl defining truss-specific abstractions:
- AbstractTrussTheory base type for truss theories
- SimpleTruss concrete theory (axial force only, pin-jointed)
- TrussFormulation{Theory} parameterized formulation struct
SimpleTruss supports 2D/3D displacement fields with 2 or 3 DOFs per node.
Documented for future extension with CableTruss and PretensionedTruss.
Part of systematic modular API architecture where each structural
element type owns its formulation abstractions.
Jukka Aho
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