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.

tests/test_error_native.rc

@@ -0,0 +1,12 @@
+int process_string(char* text) {
+    int len = strlen(text);
+    int pos = strpos(text, bad_variable);
+    return pos;
+}
+
+int main() {
+    char* message = "Hello World";
+    int result = process_string(message);
+    printf("Result: %d\n", result);
+    return 0;
+}

tests/test_error_stacktrace.rc

@@ -0,0 +1,22 @@
+int level3() {
+    int bad = undefined_var;
+    return bad;
+}
+
+int level2(int x) {
+    int result = level3();
+    return result + x;
+}
+
+int level1(int a, int b) {
+    int sum = a + b;
+    int final = level2(sum);
+    return final;
+}
+
+int main() {
+    printf("Starting program\n");
+    int value = level1(5, 10);
+    printf("Value: %d\n", value);
+    return 0;
+}

tests/test_error_undefined_var.rc

@@ -0,0 +1,6 @@
+int main() {
+    int x = 10;
+    int y = unknown_variable + 5;
+    printf("Y: %d\n", y);
+    return 0;
+}