Refactor: extract lib.rs into focused sub-modules, add --output-dir CLI option

- Split monolithic src/lib.rs (~640 lines) into 7 sub-modules:
  types.rs, context.rs, user_data.rs, translate.rs, http.rs,
  csv_util.rs, test_utils.rs
- Add configurable --output-dir CLI option for CSV output files
- Implement atomic file writes (write-to-temp + rename) for all CSVs
- Change compute_car_counts() to return CarCounts directly (not Result)
- Add test-utils feature gate for shared test helpers
- Add HTTP mock tests with mockito (6 tests)
- Add proptest tests for compute_car_counts (3 property tests)
- Update integration tests to use shared test utilities
- Update documentation (API.md, ARCHITECTURE.md, DATA_STRUCTURES.md)

src/parsing/mod.rs

@@ -364,22 +364,7 @@ async fn fetch_car(rid: &str, times_dir: &str) -> Result<(), AppError> {
 mod tests {
     use super::*;
     use std::collections::HashSet;
-
-    // -- Helper: create a test BestCarRecord --
-    fn make_record(car_rid: &str, rq: u8, time: f64, guid: &str) -> BestCarRecord {
-        BestCarRecord {
-            car_rid: car_rid.to_string(),
-            brand: "TestBrand".to_string(),
-            only_name: "TestModel".to_string(),
-            year: 2020,
-            rq,
-            config: "111".to_string(),
-            time,
-            owned: false,
-            prize: false,
-            guid: guid.to_string(),
-        }
-    }
+    use top_drives::test_utils::make_record;
 
     // =======================================================================
     // update_best_car tests