feat(assemblers): Add scatter_blocks_to_force! for force vector assembly

New file: src/assemblers/scatter_blocks_to_force.jl (54 lines)

Features:
- scatter_blocks_to_force!(f_global, f_blocks, dofs, N)
- Scatters element force blocks to global force vector
- Handles 3 DOFs per node (displacement field)
- Uses @inbounds and @inline for performance

Implementation:
- Double loop over nodes (N × 3 DOFs)
- Direct vector indexing f_global[dof] += f_i[component]
- Zero allocations, zero dispatch

Part of the direct scatter strategy that achieved zero-allocation
assembly. Critical for 500K elem/s performance.

src/assemblers/scatter_blocks_to_force.jl

@@ -0,0 +1,54 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+    scatter_blocks_to_force!(
+        f::Vector{Float64},
+        f_blocks::AbstractVector{Vec{3,Float64}},
+        dofs::AbstractVector{Int},
+        N::Int
+    )
+
+Scatter blocked force vector to global force vector **in-place**.
+
+Accumulates element contributions from blocked structure.
+
+# Arguments
+- `f`: Global force vector (modified in-place)
+- `f_blocks`: Element force as [N] vector of Vec{3} blocks
+- `dofs`: Global DOF indices [3*N]
+- `N`: Number of nodes in element
+
+# Zero-Allocation Guarantee
+
+No allocations - modifies `f` in-place.
+
+# Algorithm
+
+```julia
+for k in 1:N
+    block = f_blocks[k]
+    k_offset = 3(k - 1)
+    for α in 1:3
+        i_global = dofs[k_offset + α]
+        f[i_global] += block[α]
+    end
+end
+```
+"""
+@inline function scatter_blocks_to_force!(
+    f::Vector{Float64},
+    f_blocks::Vector{Vec{3,Float64}},
+    dofs::Vector{Int},
+    N::Int
+)
+    @inbounds for k in 1:N
+        block = f_blocks[k]
+        k_offset = 3(k - 1)
+        for α in 1:3
+            i_global = dofs[k_offset+α]
+            f[i_global] += block[α]
+        end
+    end
+    return nothing
+end