fix(M03): support cold rendered ground sample startup

src/tools/verify_rendered_ground_sample.gd

@@ -0,0 +1,45 @@
+extends SceneTree
+
+## Cold-start contract for the renderer-owned ground query result value.
+
+const SAMPLE_SCRIPT := preload("res://src/render/terrain/rendered_ground_sample.gd")
+
+
+func _initialize() -> void:
+	var failures: Array[String] = []
+	var available_sample: RefCounted = SAMPLE_SCRIPT.available(42.25)
+	_expect_true(bool(available_sample.get("is_available")), "available sample", failures)
+	_expect_near(float(available_sample.get("height_units")), 42.25, "available height", failures)
+
+	var non_finite_sample: RefCounted = SAMPLE_SCRIPT.available(INF)
+	_expect_true(not bool(non_finite_sample.get("is_available")), "non-finite unavailable", failures)
+	_expect_true(
+		StringName(non_finite_sample.get("failure_code")) == &"render_terrain_height_not_finite",
+		"non-finite failure code",
+		failures
+	)
+
+	var unavailable_sample: RefCounted = SAMPLE_SCRIPT.unavailable(&"")
+	_expect_true(
+		StringName(unavailable_sample.get("failure_code")) == &"render_terrain_unavailable",
+		"empty failure code normalization",
+		failures
+	)
+
+	if not failures.is_empty():
+		for failure in failures:
+			push_error("RENDERED_GROUND_SAMPLE: %s" % failure)
+		quit(1)
+		return
+	print("RENDERED_GROUND_SAMPLE PASS contract=3")
+	quit(0)
+
+
+func _expect_near(actual_value: float, expected_value: float, label: String, failures: Array[String]) -> void:
+	if not is_equal_approx(actual_value, expected_value):
+		failures.append("%s expected %.3f, got %.3f" % [label, expected_value, actual_value])
+
+
+func _expect_true(actual_value: bool, label: String, failures: Array[String]) -> void:
+	if not actual_value:
+		failures.append("%s expected true" % label)