feat(assemblers): add scatter_to_force! for force vector assembly

New 37-line scatter function for element-based force assembly:
- Scatters element force vector to global force vector in-place
- Zero-allocation guarantee (modifies global vector directly)
- Legacy format support
- Required dependency for element_based_coo.jl

Provides core force scattering functionality for COO assembly.

src/assemblers/scatter_to_force.jl

@@ -0,0 +1,37 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+    scatter_to_force!(f::Vector{Float64}, fe::AbstractVector, dofs::AbstractVector{Int})
+
+Scatter element force vector to global force vector **in-place** (legacy format).
+
+Accumulates element contributions: `f[dofs] += fe`
+
+# Arguments
+- `f`: Global force vector (modified in-place)
+- `fe`: Element force vector [ndofs_elem]
+- `dofs`: Global DOF indices [ndofs_elem]
+
+# Zero-Allocation Guarantee
+
+No allocations - modifies `f` in-place.
+
+# Algorithm
+
+```julia
+for (i_local, i_global) in enumerate(dofs)
+    f[i_global] += fe[i_local]
+end
+```
+"""
+function scatter_to_force!(
+    f::Vector{Float64},
+    fe::Vector{Float64},
+    dofs::Vector{Int}
+)
+    for (i_local, i_global) in enumerate(dofs)
+        f[i_global] += fe[i_local]
+    end
+    return nothing
+end