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/track.rs

@@ -8,10 +8,15 @@ use crate::commands::{load_my_cars_or_warn, run_analysis};
 use crate::output::print_best_car;
 
 /// Executes the track command: shows best car for a specific track/condition/RQ.
+///
+/// Note: `_output_dir` is unused because the track command prints to stdout
+/// rather than writing a CSV file. It exists for API consistency with other
+/// command signatures so the caller doesn't need per-command branching.
 pub async fn run_track(
     my_cars_path: &str,
     cars_db_path: &str,
     times_dir: &str,
+    _output_dir: &str,
     track: &str,
     condition: &str,
     rq: u8,