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/DATA_STRUCTURES.md

@@ -199,7 +199,7 @@ This document describes the JSON data structures used by the Top Drives Records
 | *value* | String | Human-readable track name (e.g., `"Laguna Seca"`) |
 
 **Usage in Application**:
-- Loaded by `translate_track_name()` in `src/lib.rs`
+- Loaded by `translate_track_name()` in `src/translate.rs`
 - Compiled into the binary at build time
 - Fallback: Returns original track code if no mapping found