test(continuum): Add tests for cache reset functions

New file: test/domains/continuum/test_reset_functions.jl

Tests for:
- reset!(cache) for COOCache
- Validates counter reset to 0
- Validates I, J, V arrays zeroed
- Validates force vector zeroed
- Validates element/geometry/material caches reset

Purpose:
- Ensure caches can be reused across multiple assemblies
- Verify no stale data remains
- Test incremental assembly workflows

Critical for iterative solvers and nonlinear problems where
assembly is repeated many times with updated state.

test/domains/continuum/test_reset_functions.jl

@@ -0,0 +1,118 @@
+# Test reset! functions for all cache types
+
+using JuliaFEM
+using Tensors
+using LinearAlgebra
+using Test
+
+include("test_helpers.jl")
+
+@testset "reset! functions" begin
+    kernel = create_test_kernel()
+    mesh = create_test_mesh()
+
+    # Create caches
+    N = 8      # Nodes per element (Hex8)
+    NIP = 8    # Integration points (Gauss{2} for Hex8)
+
+    geometry_cache = JuliaFEM.create_geometry_cache(N, NIP)
+    element_cache = JuliaFEM.create_element_cache(mesh, kernel)
+    material_cache = JuliaFEM.create_material_cache(kernel.material, NIP)
+
+    @testset "reset!(GeometryCache)" begin
+        # Populate with data first
+        elem_id = 1
+        JuliaFEM.update_geometry_cache!(geometry_cache, element_cache, kernel, elem_id, mesh)
+
+        # Verify data exists
+        @test any(x -> norm(x) > 0, geometry_cache.X)
+        @test any(x -> x != 0, geometry_cache.detJ_w)
+
+        # Reset and verify zeros
+        JuliaFEM.reset!(geometry_cache)
+
+        @test all(x -> x == zero(Vec{3,Float64}), geometry_cache.X)
+        @test all(x -> x == 0.0, geometry_cache.detJ_w)
+        for q in 1:NIP
+            @test all(x -> x == zero(Vec{3,Float64}), view(geometry_cache.∇N_data, q, :))
+        end
+
+        # Test zero allocations (warm-up first)
+        JuliaFEM.reset!(geometry_cache)
+        allocs = @allocated JuliaFEM.reset!(geometry_cache)
+        @test allocs == 0
+    end
+
+    @testset "reset!(ElementCache)" begin
+        # Populate with data first
+        elem_id = 1
+        JuliaFEM.update_element_cache!(element_cache, kernel, elem_id, mesh, nothing)
+
+        # Verify data exists
+        @test any(x -> x != 0, element_cache.dofs)
+
+        # Reset and verify zeros
+        JuliaFEM.reset!(element_cache)
+
+        @test all(x -> x == 0, element_cache.dofs)
+        @test all(x -> x == zero(Tensor{2,3,Float64}), element_cache.K_blocks)
+        @test all(x -> x == zero(Vec{3,Float64}), element_cache.f_blocks)
+        @test all(x -> x == zero(Vec{3,Float64}), element_cache.u_buffer)
+
+        # Test zero allocations (warm-up first)
+        JuliaFEM.reset!(element_cache)
+        allocs = @allocated JuliaFEM.reset!(element_cache)
+        @test allocs == 0
+    end
+
+    @testset "reset!(MaterialStateCache)" begin
+        # Populate with data first (need geometry cache)
+        elem_id = 1
+        u_global = nothing
+        state_old = create_material_state(kernel, mesh)
+        Δt = 0.01
+
+        JuliaFEM.update_geometry_cache!(geometry_cache, element_cache, kernel, elem_id, mesh)
+        JuliaFEM.update_element_cache!(element_cache, kernel, elem_id, mesh, u_global)
+        JuliaFEM.update_material_cache!(material_cache, geometry_cache, kernel.material,
+            element_cache, state_old, elem_id, Δt)
+
+        # Verify data exists (tangent should be non-zero for linear elastic)
+        @test any(q -> norm(material_cache.𝔻[q]) > 0, 1:NIP)
+
+        # Reset and verify zeros
+        JuliaFEM.reset!(material_cache)
+
+        @test all(q -> material_cache.σ[q] == zero(SymmetricTensor{2,3,Float64}), 1:NIP)
+        @test all(q -> material_cache.𝔻[q] == zero(SymmetricTensor{4,3,Float64}), 1:NIP)
+
+        # Test zero allocations (warm-up first)
+        JuliaFEM.reset!(material_cache)
+        allocs = @allocated JuliaFEM.reset!(material_cache)
+        @test allocs == 0
+    end
+
+    @testset "reset!(COOCache)" begin
+        # Create COO cache
+        assembler = COOAssembler()
+        coo_cache = create_cache(assembler, mesh, kernel)
+
+        # Populate with data by assembling
+        assemble!(coo_cache, assembler, kernel, mesh)
+
+        # Verify data exists (counter should be > 0 after assembly)
+        @test coo_cache.counter[] > 0
+        # Note: force vector f is zero for zero displacement with no body forces
+
+        # Reset and verify zeros
+        JuliaFEM.reset!(coo_cache)
+
+        @test coo_cache.counter[] == 0
+        @test all(x -> x == 0.0, coo_cache.f)
+
+        # Test zero allocations (warm-up first)
+        JuliaFEM.reset!(coo_cache)
+        allocs = @allocated JuliaFEM.reset!(coo_cache)
+        @test allocs == 0
+    end
+end