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/API.md

@@ -8,7 +8,14 @@ This document provides comprehensive API documentation for the Top Drives Record
 
 ```
 src/
-├── lib.rs                 # Shared library: types, utilities, error handling
+├── lib.rs                 # Thin re-export module: declares sub-modules, re-exports public types
+├── types.rs               # Core types: AppError, Car, BestCarRecord, TripletKey
+├── context.rs             # Analysis state: AnalysisContext, CarCounts, compute_car_counts
+├── user_data.rs           # User data: MyCarsData, load_my_cars_data
+├── translate.rs           # Translation: translate_track_name, translate_track_type
+├── http.rs                # HTTP: get_file (async download with retry)
+├── csv_util.rs            # CSV utilities: compare_cars, escape_csv_field
+├── test_utils.rs          # Test helpers: make_record, make_car
 ├── records.rs             # Binary entry point: CLI definition and dispatch
 ├── commands/
 │   ├── mod.rs             # Command dispatch, run_analysis(), load_my_cars_or_warn()
@@ -30,7 +37,7 @@ src/
 
 ## 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.
 
 ### Public Types
 
@@ -176,7 +183,7 @@ pub enum AppError {
 
 - Sorts by: RQ → Brand → Model name
 
-#### `compute_car_counts(ctx: &AnalysisContext, car_list: Vec<Car>) -> Result<CarCounts, AppError>`
+#### `compute_car_counts(ctx: &AnalysisContext, car_list: Vec<Car>) -> CarCounts`
 **Aggregates per-car win counts from analysis context**
 
 - Iterates all entries in the `DashMap`