test-infra: preflight daemon/compose checks + Colima runtime docs

The infra previously assumed Docker Desktop: an implicitly running
daemon and bundled CLI plugins. On the Colima runtime (free OSS,
CLI-first) neither is implicit, and a missing daemon surfaced as
cryptic compose errors. run.sh now fails fast with the exact setup
commands (colima start with --vz-rosetta for fast amd64 legs, plugin
symlinks for the brew docker client) and documents the runtime in the
header.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

test-infrastructure/run.sh

@@ -30,11 +30,31 @@
 #   ./test-infrastructure/run.sh lint         # clang-format + cppcheck
 #   ./test-infrastructure/run.sh shell        # debug shell
 
+# Runtime: any Docker-compatible daemon. On macOS we use Colima (free OSS):
+#   brew install colima docker docker-compose docker-buildx
+#   ln -sf /opt/homebrew/opt/docker-compose/bin/docker-compose ~/.docker/cli-plugins/docker-compose
+#   ln -sf /opt/homebrew/opt/docker-buildx/bin/docker-buildx  ~/.docker/cli-plugins/docker-buildx
+#   colima start --vm-type vz --vz-rosetta --cpu 12 --memory 16
+# --vz-rosetta is required for fast amd64 legs (QEMU otherwise). Autostart at
+# login: brew services start colima.
+
 set -euo pipefail
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd)"
 COMPOSE="docker compose -f $ROOT/test-infrastructure/docker-compose.yml"
 
+if ! docker info >/dev/null 2>&1; then
+    echo "ERROR: no Docker daemon reachable." >&2
+    echo "  Start one first — on macOS: colima start --vm-type vz --vz-rosetta --cpu 12 --memory 16" >&2
+    echo "  (current context: $(docker context show 2>/dev/null || echo unknown))" >&2
+    exit 1
+fi
+if ! docker compose version >/dev/null 2>&1; then
+    echo "ERROR: docker compose plugin missing." >&2
+    echo "  brew install docker-compose && ln -sf /opt/homebrew/opt/docker-compose/bin/docker-compose ~/.docker/cli-plugins/docker-compose" >&2
+    exit 1
+fi
+
 case "${1:-full}" in
     full)
         echo "=== Linux arm64: test + build ==="