chore(assemblers): add shared redundant-kernel depwarn helper

Introduce _depwarn_redundant_kernel_arg! and a session Ref so legacy overloads that ignore a volume kernel emit a single Base.depwarn toward kernel-column-only APIs.

src/assemblers/redundant_kernel_depwarn.jl

@@ -0,0 +1,23 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+# One `Base.depwarn` per session for overloads that take a redundant volume `kernel`
+# (ignored in favour of `cache.kernel_column` on the DOF-based path, or unused in
+# symmetric Neumann `apply_load!` signatures).
+const _REDUNDANT_KERNEL_ARG_DEPWARN = Ref(false)
+
+@noinline function _depwarn_redundant_kernel_arg!(caller::Symbol)
+    if !_REDUNDANT_KERNEL_ARG_DEPWARN[]
+        _REDUNDANT_KERNEL_ARG_DEPWARN[] = true
+        Base.depwarn(
+            "overload with a redundant `kernel` argument is deprecated (it is not read; " *
+            "DOF-based volume assembly reads kernels from `cache.kernel_column`). " *
+            "Prefer `(cache, asm, mesh; …)`, `(y, cache, asm, mesh, x; …)`, and the same " *
+            "pattern for matrix-free operators, preconditioners, eigensolve helpers, " *
+            "partitioned matvec / MPI drivers, and `compute_block_diagonal!` / " *
+            "`BlockJacobiPreconditioner` entry points without a separate volume kernel.",
+            caller,
+        )
+    end
+    return nothing
+end