fix: add bounds checks and robustness improvements to interpreter, parser, and tokenizer

Makefile

@@ -137,6 +137,9 @@ test-eval: $(TARGET)
 test-validation: $(TARGET)
 	$(TARGET) $(TEST_DIR)/test_input_validation.rc
 
+test-robustness: $(TARGET)
+	$(TARGET) $(TEST_DIR)/test_robustness.rc
+
 demo-error-stacktrace: $(TARGET)
 	@echo "=== Demonstrating Full Stack Trace ==="
 	-$(TARGET) $(TEST_DIR)/test_error_stacktrace.rc
@@ -182,6 +185,7 @@ help:
 	@echo "  make test-preprocessor - Preprocessor (2 tests)"
 	@echo "  make test-eval        - Eval function (30 tests)"
 	@echo "  make test-validation  - Input validation (77 tests)"
+	@echo "  make test-robustness  - Robustness and safety (25 tests)"
 	@echo ""
 	@echo "Error Handling Demonstrations:"
 	@echo "  make demo-errors              - Run all error handling demos"