chore(physics): delete legacy microkernel placeholders

Remove early microkernel scaffolding superseded by domain-local kernels.

- Drop `src/physics/microkernels.jl`.

src/physics/microkernels.jl

@@ -1,82 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
-
-"""
-Microkernel interface for DOF-based assembly.
-
-Enables multi-physics coupling via type dispatch, matrix-free solvers,
-and zero-allocation assembly.
-
-See `docs/src/developer/microkernel_architecture.md` for complete documentation.
-"""
-
-using ..JuliaFEM: AbstractField, AbstractKernel
-using ..JuliaFEM: GeometryCache, AssemblyMaterialWorkspace
-
-# ============================================================================
-# MICROKERNEL INTERFACE
-# ============================================================================
-
-"""
-    evaluate(kernel, field_i, field_j, k, l, comp_i, comp_j, material_cache, geometry_cache, q) -> Float64
-
-Compute single scalar contribution to K[i,j] at integration point q.
-
-# Arguments
-- `kernel`: Kernel instance (holds material, geometry properties)
-- `field_i`, `field_j`: Field types for dispatch (Displacement, Temperature, etc.)
-- `k`, `l`: Shape function/node indices
-- `comp_i`, `comp_j`: Component indices (1-3 for vectors, 1 for scalars)
-- `material_cache`: Precomputed material state (σ, 𝔻, κ, etc.)
-- `geometry_cache`: Precomputed geometry (∇N, detJ, etc.)
-- `q`: Integration point index
-
-# Returns
-Float64 value WITHOUT detJ*w scaling (assembler handles weighting).
-
-Default implementation returns 0.0 (no coupling). Override for specific kernel-field pairs.
-
-See `docs/src/developer/microkernel_architecture.md` for design rationale and examples.
-"""
-@inline function evaluate(
-    kernel::AbstractKernel,
-    field_i::AbstractField,
-    field_j::AbstractField,
-    k::Int,
-    l::Int,
-    comp_i::Int,
-    comp_j::Int,
-    material_workspace::AssemblyMaterialWorkspace,
-    geometry_cache::GeometryCache,
-    q::Int
-)
-    return 0.0  # Default: no coupling
-end
-
-# ============================================================================
-# TRAITS FOR KERNEL REQUIREMENTS
-# ============================================================================
-
-"""
-    requires_basis_values(kernel) -> Bool
-
-Does kernel need basis function VALUES (N)?
-Default: false (most kernels only need gradients).
-"""
-requires_basis_values(::AbstractKernel) = false
-
-"""
-    requires_basis_gradients(kernel) -> Bool
-
-Does kernel need basis function GRADIENTS (∇N)?
-Default: true (most kernels need gradients).
-"""
-requires_basis_gradients(::AbstractKernel) = true
-
-"""
-    requires_basis_second_derivatives(kernel) -> Bool
-
-Does kernel need basis function SECOND DERIVATIVES (∇²N)?
-Default: false (only beam/plate bending needs this).
-"""
-requires_basis_second_derivatives(::AbstractKernel) = false