feat(assets): seven asset commands + Run dispatch/helpers

check-dupes, clean-dupes, check-mdl, fix-mdl, convert, upscale, compile
behind a single runner indirection, mirroring internal/depot's Run shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/assets/run_test.go

@@ -0,0 +1,26 @@
+package assets
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func env(string) string { return "" }
+
+func TestRunNoArgsIsUsage(t *testing.T) {
+	var out, errw bytes.Buffer
+	if code := Run(nil, &out, &errw, env); code != exitUsage {
+		t.Fatalf("no args exit = %d, want %d", code, exitUsage)
+	}
+	if !strings.Contains(errw.String(), "usage") {
+		t.Fatalf("no args should print usage, got: %q", errw.String())
+	}
+}
+
+func TestRunUnknownSubcommandIsUsage(t *testing.T) {
+	var out, errw bytes.Buffer
+	if code := Run([]string{"frobnicate"}, &out, &errw, env); code != exitUsage {
+		t.Fatalf("unknown subcommand exit = %d, want %d", code, exitUsage)
+	}
+}