Take the catalogue's composition rules into the node-engine

ADR 0593 §7: the composer with its refusals leaves mesh-sdk, so mesh-sdk
carries no composition rule. The catalogue functions only the
node-engine's composition reaches (Resolve, Resolution's Compose,
Declaration, DataItems, LeftOut, Rules and the rest, the settings,
placeholder and template refusals, the rendering helpers) are copied
with their tests into internal/compose/resolution; a method of a
mesh-sdk shape becomes a function taking it first. The composition
calls its own copies, and the captured catalogue its tests read lives
here now, shared by mesh-compose's tests.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

internal/compose/resolution/environment_items_test.go

@@ -0,0 +1,19 @@
+package resolution
+
+import (
+	"strings"
+	"testing"
+
+	mcat "git.novox.be/novox/mesh-sdk/go/catalogue"
+)
+
+func TestTwoModulesSettingOneVariableAreRefused(t *testing.T) {
+	// A module that sets a variable twice, in two modules, is refused as the old form refused it.
+	a := mcat.Manifest{Module: "a", Items: []mcat.Item{{Collection: "environment", Content: "1\n",
+		Attributes: map[string]string{"kind": "variable", "name": "X"}}}}
+	b := mcat.Manifest{Module: "b", Items: []mcat.Item{{Collection: "environment", Content: "2\n",
+		Attributes: map[string]string{"kind": "variable", "name": "X"}}}}
+	if err := variablesSetOnce([]mcat.Manifest{a, b}); err == nil || !strings.Contains(err.Error(), "both set X") {
+		t.Errorf("two modules setting X: %v", err)
+	}
+}