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/commands/mod.rs

@@ -58,6 +58,6 @@ pub async fn run_analysis(
     let ctx = AnalysisContext::new();
     let owned = Arc::new(owned.clone());
     let car_list = parse_all_times(owned, &ctx, cars_db_path, times_dir).await?;
-    let counts = compute_car_counts(&ctx, car_list)?;
+    let counts = compute_car_counts(&ctx, car_list);
     Ok((ctx, counts))
 }