refactor(util): move panel version comparison into a shared package

The node-assignment path in the service package needs the same
MAJOR.MINOR.PATCH comparison the panel updater uses, but service/panel
imports service, so the helper cannot stay there.

internal/util/version/version_test.go

@@ -0,0 +1,12 @@
+package version
+
+import "testing"
+
+func TestCompareRejectsUnexpectedFormats(t *testing.T) {
+	if _, ok := Compare("latest", "2.9.3"); ok {
+		t.Fatal("expected non-semver latest tag to be rejected")
+	}
+	if _, ok := Compare("v2.9", "2.9.3"); ok {
+		t.Fatal("expected short version to be rejected")
+	}
+}