Add staggered HDCG multigrid test and mac-arm Homebrew build scripts

Test_staggered_hdcg.cc implements a two-level ADEF2 multigrid solver for
NaiveStaggeredFermion using SchurStaggeredOperator, following the mrhs
hermitian multigrid approach of arXiv:2409.03904. Uses a 33-point coarse
stencil (NextToNearestStencilGeometry4D) with nbasis=24, block={4,4,4,4},
and Chebyshev subspace generation with hi=5.0 (lambda_max ~4.6).

Also adds systems/mac-arm/sourceme-homebrew.sh and config-command-homebrew
for building Grid on Apple Silicon with Homebrew-installed dependencies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

systems/mac-arm/config-command-homebrew

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Configure Grid from a build directory two levels below the source root,
+# e.g.: mkdir -p systems/mac-arm/build && cd systems/mac-arm/build && bash ../config-command-homebrew
+#
+# Prerequisites: source sourceme-homebrew.sh first.
+
+CXX=mpicxx ../../configure \
+    --enable-simd=GEN \
+    --enable-comms=mpi-auto \
+    --enable-Sp=yes \
+    --enable-unified=yes \
+    --prefix="${HOME}/Grid-install" \
+    --with-lime="${CLIME}" \
+    --with-openssl="${OPENSSL}" \
+    --with-gmp="${GMP}" \
+    --with-mpfr="${MPFR}" \
+    --with-fftw="${FFTW}" \
+    --disable-debug

systems/mac-arm/sourceme-homebrew.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# Environment for building Grid on Apple Silicon Mac with Homebrew dependencies.
+# Usage: source systems/mac-arm/sourceme-homebrew.sh
+
+HOMEBREW=/opt/homebrew
+
+export GMP=${HOMEBREW}/opt/gmp
+export MPFR=${HOMEBREW}/opt/mpfr
+export FFTW=${HOMEBREW}/opt/fftw
+export OPENSSL=${HOMEBREW}/opt/openssl@3
+export CLIME=/usr/local
+
+export PATH="${HOMEBREW}/opt/open-mpi/bin:${HOMEBREW}/bin:${PATH}"
+export LDFLAGS="-L${OPENSSL}/lib"
+export CPPFLAGS="-I${OPENSSL}/include"