feat(trusses): Add truss formulation API with SimpleTruss theory

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.

src/trusses/api.jl

@@ -0,0 +1,79 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Truss formulation API definitions.
+
+This file defines truss-specific abstract types and formulation theories.
+Must be included after core api.jl.
+"""
+
+# ============================================================================
+# TRUSS FORMULATION THEORIES
+# ============================================================================
+
+"""
+    AbstractTrussTheory
+
+Abstract type for truss theory variants.
+
+Truss elements carry only axial forces (tension/compression), no bending.
+
+# Concrete Theories
+- `SimpleTruss`: Standard 2-node truss (axial force only)
+
+# Future Extensions
+- `CableTruss`: Cable elements (tension only, no compression)
+- `PretensionedTruss`: Trusses with initial stress
+
+# See Also
+- [`TrussFormulation`](@ref)
+"""
+abstract type AbstractTrussTheory end
+
+"""
+    SimpleTruss <: AbstractTrussTheory
+
+Simple truss theory (axial force only).
+
+Assumptions:
+- Only axial forces (tension/compression)
+- No bending moments
+- Pin-jointed connections
+- 3 DOFs per node in 3D: (ux, uy, uz)
+- 2 DOFs per node in 2D: (ux, uy)
+
+# Usage
+```julia
+formulation = TrussFormulation{SimpleTruss}()
+physics = Physics(
+    formulation=formulation,
+    field=Displacement{3}(),  # 3D truss
+    mesh=truss_mesh,
+    material=steel
+)
+```
+"""
+struct SimpleTruss <: AbstractTrussTheory end
+
+"""
+    TrussFormulation{Theory<:AbstractTrussTheory} <: AbstractFormulation
+
+Truss element formulation with theory variant.
+
+# Type Parameter
+- `Theory`: Truss theory type (SimpleTruss, CableTruss, etc.)
+
+# Examples
+```julia
+# Standard truss
+TrussFormulation{SimpleTruss}()
+```
+
+# Fields per Node
+- 3D: 3 DOFs (ux, uy, uz)
+- 2D: 2 DOFs (ux, uy)
+
+Use with `Displacement{Dim}` field type.
+"""
+struct TrussFormulation{Theory<:AbstractTrussTheory} <: AbstractFormulation end