Improve test coverage for Issue #78 fix

- Remove unused dense_original.jl reference file
- Add tests for type stability helper functions
- Add tests for broadcast type stability
- Coverage now includes _stabilize_mesh_type, _create_mesharray_typed,
  _infer_mesh_type, and _copyto_typed!

Author: hsugawa8651

src/mesharrays/dense_original.jl

@@ -94,3 +94,54 @@ end
     g = MeshArray(mesh1, mesh1, mesh1, mesh1, mesh1, mesh1)
     @test isconcretetype(typeof(g.mesh))
 end
+
+@testset "MeshArray Type Stability Helpers (Issue #78)" begin
+    N1, N2 = 8, 6
+    mesh1 = SimpleGrid.Uniform{Float64}([0.0, 1.0], N1)
+    mesh2 = SimpleGrid.Uniform{Float64}([0.0, 1.0], N2)
+
+    mesh_tuple = (mesh1, mesh2)
+    stabilized = MeshArrays._stabilize_mesh_type(mesh_tuple)
+    @test isconcretetype(typeof(stabilized))
+    @test stabilized === mesh_tuple
+
+    non_concrete_mesh = Any[mesh1, mesh2]
+    stabilized_from_vec = MeshArrays._stabilize_mesh_type(non_concrete_mesh)
+    @test isconcretetype(typeof(stabilized_from_vec))
+
+    data = rand(N1, N2)
+    result = MeshArrays._create_mesharray_typed(data, mesh_tuple, Float64, 2)
+    @test result isa MeshArray{Float64, 2}
+    @test result.data === data
+
+    data_int = ones(Int, N1, N2)
+    result_converted = MeshArrays._create_mesharray_typed(data_int, mesh_tuple, Float64, 2)
+    @test result_converted isa MeshArray{Float64, 2}
+    @test eltype(result_converted.data) == Float64
+
+    MT = typeof(mesh_tuple)
+    @test MeshArrays._infer_mesh_type(mesh_tuple) == MT
+end
+
+@testset "MeshArray Broadcast Type Stability" begin
+    N1, N2 = 10, 12
+    mesh1 = SimpleGrid.Uniform{Float64}([0.0, 1.0], N1)
+    mesh2 = SimpleGrid.Uniform{Float64}([0.0, 1.0], N2)
+
+    g1 = MeshArray(mesh1, mesh2; data=rand(N1, N2))
+    g2 = MeshArray(mesh1, mesh2; data=rand(N1, N2))
+
+    g3 = similar(g1)
+    g3 .= g1 .+ g2
+    @test g3.data ≈ g1.data .+ g2.data
+
+    g4 = similar(g1)
+    Base.copyto!(g4, Base.Broadcast.broadcasted(+, g1, g2))
+    @test g4.data ≈ g1.data .+ g2.data
+
+    indices = CartesianIndices(g1.data)
+    bcf = Base.Broadcast.flatten(Base.Broadcast.broadcasted(*, g1, 2.0))
+    g5 = similar(g1)
+    MeshArrays._copyto_typed!(g5, bcf, indices)
+    @test g5.data ≈ g1.data .* 2.0
+end