feat: add call stack tracking and enhanced error reporting with file:line context

Implement a full call stack system that tracks function calls during interpretation, including native function boundaries. Add `push_call_frame`/`pop_call_frame` functions, a `CallStack` struct with depth-limited frames, and `print_stacktrace` for formatted error output. Introduce `error_with_context` to replace bare `error()` calls, and modify the tokenizer to track line/column/filename per token. Update the parser and interpreter to push/pop frames on function entry/exit, and add new Makefile targets (`demo-error-*`) plus test scripts (`test_error_stacktrace.rc`, `test_error_native.rc`, `test_error_undefined_var.rc`) and an `examples/error_handling_demo.rc` to demonstrate the new error reporting capabilities.

examples/error_handling_demo.rc

@@ -0,0 +1,28 @@
+int deep_function_3() {
+    int result = non_existent_variable * 5;
+    return result;
+}
+
+int deep_function_2(int x) {
+    printf("In deep_function_2 with x=%d\n", x);
+    int value = deep_function_3();
+    return value + x;
+}
+
+int deep_function_1(int a, int b) {
+    printf("In deep_function_1 with a=%d, b=%d\n", a, b);
+    int sum = a + b;
+    int result = deep_function_2(sum);
+    return result;
+}
+
+int main() {
+    printf("=== Error Handling Demonstration ===\n");
+    printf("This program will intentionally trigger an error\n");
+    printf("to demonstrate the enhanced error reporting system.\n\n");
+
+    int final_result = deep_function_1(10, 20);
+    printf("Final result: %d\n", final_result);
+
+    return 0;
+}