chore(tooling): add GUT test infrastructure and smoke test

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

.gutconfig.json

@@ -0,0 +1,8 @@
+{
+  "dirs": ["res://test/"],
+  "prefix": "test_",
+  "suffix": ".gd",
+  "include_subdirs": true,
+  "log_level": 1,
+  "export_path": ""
+}

test/unit/test_smoke.gd

@@ -0,0 +1,10 @@
+## Smoke test — verifies GUT is installed and the test runner works.
+extends GutTest
+
+
+func test_gut_is_working() -> void:
+	assert_true(true)
+
+
+func test_basic_arithmetic() -> void:
+	assert_eq(1 + 1, 2)