Add parse_program to parser

The `Parser` can now parse a sequence of top-level expressions,
representing a complete program. This change modifies the `roundtrip`
function to iterate over these expressions and emit them, allowing for
multi-line program parsing.

tests/data_server.rs

@@ -120,6 +120,7 @@ fn test_real_m1_full_iteration() {
     assert!(total_records > 0, "Should have read some records");
 }
 
+// FIXME: This test is flaky — thread scheduling can cause uneven read counts.
 #[test]
 fn test_real_concurrent_access() {
     if skip_if_no_data() {