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)

Cargo.toml

@@ -21,6 +21,12 @@ clap = { version = "4", features = ["derive"] }
 thiserror = "2"
 dashmap = "6"
 
+[features]
+default = []
+test-utils = []
+
 [dev-dependencies]
+top_drives = { path = ".", features = ["test-utils"] }
 tempfile = "3.10.1"
 proptest = "1"
+mockito = "1"