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)

docs/ARCHITECTURE.md

@@ -8,17 +8,19 @@ The Top Drives Records Analyzer is a Rust CLI tool for analyzing car performance
 
 ### 1. Shared Library (`src/lib.rs`)
 
-**Common types and utilities** shared across the project.
+**Thin re-export module** that declares 6 focused sub-modules and re-exports all public types for backward compatibility. External code can use `top_drives::Car` without knowing the internal module layout.
 
-#### Key Responsibilities
-- **Data Types**: Defines `Car`, `BestCarRecord`, `AnalysisContext`, `CarCounts`, `MyCarsData`, `AppError`
-- **Track Name Translation**: Converts track codes to human-readable names via `translate_track_name()`
-- **Track Type Translation**: Converts condition codes to surface descriptions via `translate_track_type()`
-- **File Download**: Async file download utility with retry logic via `get_file()`
-- **User Data Loading**: Parses `my_cars.json` via `load_my_cars_data()`
-- **Car Comparison**: Sorting comparator via `compare_cars()`
-- **Count Aggregation**: Per-car win count computation via `compute_car_counts()`
-- **CSV Escaping**: Safe CSV field formatting via `escape_csv_field()`
+#### Sub-modules
+
+| Module | File | Key Exports |
+|--------|------|-------------|
+| `types` | `src/types.rs` | `AppError`, `BestCarRecord`, `Car`, `TripletKey` |
+| `context` | `src/context.rs` | `AnalysisContext`, `CarCounts`, `compute_car_counts` |
+| `user_data` | `src/user_data.rs` | `MyCarsData`, `load_my_cars_data` |
+| `translate` | `src/translate.rs` | `translate_track_name`, `translate_track_type` |
+| `http` | `src/http.rs` | `get_file` |
+| `csv_util` | `src/csv_util.rs` | `compare_cars`, `escape_csv_field` |
+| `test_utils` | `src/test_utils.rs` | `make_record`, `make_car` (test helpers) |
 
 #### Error Handling
 All public functions return `Result<T, AppError>` where `AppError` is a unified error type:
@@ -42,6 +44,7 @@ All public functions return `Result<T, AppError>` where `AppError` is a unified
 - `--cars-db` — Path to car database file (default: `cars_repo.json`)
 - `--times-dir` — Directory for per-car timing JSON files (default: `times`)
 - `--tracks-db` — Path to tracks database file (default: `tracks_repo.json`)
+- `--output-dir` — Directory for output CSV files (default: `.`)
 
 ### 3. Commands Module (`src/commands/`)