feat(assemblers): add scatter_to_triplets! for COO assembly

New 63-line scatter function for element-based COO assembly:
- Scatters element stiffness matrix to triplet arrays (I, J, V)
- In-place operation with zero-allocation guarantee
- Capacity checking to prevent overflow
- Required dependency for element_based_coo.jl

Provides core scattering functionality for COO format assembly.

src/assemblers/scatter_to_triplets.jl

@@ -0,0 +1,63 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+    scatter_to_triplets!(cache::COOCache, Ke::AbstractMatrix, dofs::AbstractVector{Int})
+
+Scatter element stiffness matrix to triplet arrays **in-place**.
+
+Appends all (i, j, value) triplets from element matrix to global triplet arrays.
+Updates counter to track current position.
+
+# Arguments
+- `cache`: COO cache with triplet arrays
+- `Ke`: Element stiffness matrix [ndofs_elem × ndofs_elem]
+- `dofs`: Global DOF indices [ndofs_elem]
+
+# Zero-Allocation Guarantee
+
+Writes to pre-allocated triplet arrays. No new arrays created.
+
+# Algorithm
+
+```julia
+for (i_local, i_global) in enumerate(dofs)
+    for (j_local, j_global) in enumerate(dofs)
+        counter += 1
+        I[counter] = i_global
+        J[counter] = j_global
+        V[counter] = Ke[i_local, j_local]
+    end
+end
+```
+"""
+function scatter_to_triplets!(
+    cache::COOCache,
+    Ke::Matrix{Float64},
+    dofs::Vector{Int}
+)
+    ndofs_elem = length(dofs)
+    counter = cache.counter[]
+
+    # Check capacity
+    new_triplets = ndofs_elem * ndofs_elem
+    if counter + new_triplets > cache.capacity
+        error("COO cache overflow: need $(counter + new_triplets) triplets, " *
+              "capacity is $(cache.capacity). Increase cache size.")
+    end
+
+    # Scatter element matrix to triplets
+    for j_local in 1:ndofs_elem
+        j_global = dofs[j_local]
+        for i_local in 1:ndofs_elem
+            i_global = dofs[i_local]
+            counter += 1
+            cache.I[counter] = i_global
+            cache.J[counter] = j_global
+            cache.V[counter] = Ke[i_local, j_local]
+        end
+    end
+
+    cache.counter[] = counter
+    return nothing
+end