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/details.rs

@@ -9,10 +9,10 @@ use crate::commands::{load_my_cars_or_warn, run_analysis};
 use crate::output::write_details_csv;
 
 /// Executes the details command: generates details.csv.
-pub async fn run_details(my_cars_path: &str, cars_db_path: &str, times_dir: &str) -> Result<(), AppError> {
+pub async fn run_details(my_cars_path: &str, cars_db_path: &str, times_dir: &str, output_dir: &str) -> Result<(), AppError> {
     let my_data = load_my_cars_or_warn(my_cars_path)?;
     let (ctx, _counts) = run_analysis(&my_data.owned, cars_db_path, times_dir).await?;
-    write_details_csv(&ctx)?;
+    write_details_csv(output_dir, &ctx)?;
     println!("Written: details.csv");
     Ok(())
 }