feat(physics): Implement boundary condition methods

- Implement add_dirichlet! for essential BCs (prescribed values)
- Implement add_neumann! for natural BCs (forces/tractions)
- Support multiple nodes and DOF components in single call
- Store BCs in physics.bc_dirichlet and physics.bc_neumann
- Add usage examples for common BC patterns
- 86 lines with complete method implementations

src/physics/boundary_conditions.jl

@@ -0,0 +1,86 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Boundary condition implementations for Physics.
+
+This file provides concrete implementations of the boundary condition interface
+functions defined in `src/physics/api.jl` for the `Physics` type.
+"""
+
+# ============================================================================
+# BOUNDARY CONDITION METHODS (implement generic API)
+# ============================================================================
+
+"""
+    add_dirichlet!(physics::Physics, node_ids::Vector{Int}, components::Vector{Int}, value::Float64)
+
+Concrete implementation of Dirichlet BC application.
+
+Applies essential (prescribed) boundary conditions to specified nodes and DOF components.
+
+# Arguments
+- `physics::Physics`: Physics problem
+- `node_ids::Vector{Int}`: Node IDs where BC applies
+- `components::Vector{Int}`: Which DOF components (e.g., [1,2,3] for all, [3] for z-direction)
+- `value::Float64`: Prescribed value
+
+# Examples
+
+```julia
+# Fix all DOFs at nodes 1, 2, 3
+add_dirichlet!(physics, [1,2,3], [1,2,3], 0.0)
+
+# Fix only z-displacement at node 10
+add_dirichlet!(physics, [10], [3], 0.0)
+
+# Prescribe x-displacement at node 20
+add_dirichlet!(physics, [20], [1], 0.1)
+```
+
+See generic documentation in `src/physics/api.jl`.
+"""
+function add_dirichlet!(physics::Physics, node_ids::Vector{Int}, components::Vector{Int}, value::Float64)
+    bc = physics.bc_dirichlet
+    for node in node_ids
+        push!(bc.node_ids, node)
+        push!(bc.components, components)
+        push!(bc.values, value)
+    end
+    return nothing
+end
+
+"""
+    add_neumann!(physics::Physics, surface_ids::Vector{Int}, traction::Vec{3,Float64})
+
+Concrete implementation of Neumann BC application.
+
+Applies natural (force/traction) boundary conditions to specified surfaces.
+
+# Arguments
+- `physics::Physics`: Physics problem
+- `surface_ids::Vector{Int}`: Surface/edge element IDs where BC applies
+- `traction::Vec{3,Float64}`: Traction vector
+
+# Examples
+
+```julia
+# Apply traction to surface
+traction = Vec{3}(0.0, -1000.0, 0.0)  # 1000 N/m² downward
+add_neumann!(physics, surface_elements, traction)
+
+# Apply force to individual nodes (surface_ids are node IDs in this case)
+force = Vec{3}(0.0, -100.0, 0.0)  # 100 N downward
+add_neumann!(physics, loaded_nodes, force)
+```
+
+See generic documentation in `src/physics/api.jl`.
+"""
+function add_neumann!(physics::Physics, surface_ids::Vector{Int}, traction::Vec{3,Float64})
+    bc = physics.bc_neumann
+    for surf in surface_ids
+        push!(bc.surface_ids, surf)
+        push!(bc.values, traction)
+    end
+    return nothing
+end