refactor: modular architecture with unified error handling and DashMap concurrency

- Break monolithic records.rs into commands/, output/, parsing/ modules
- Add lib.rs with shared types (AppError, Car, BestCarRecord, etc.)
- Replace Arc<Mutex<HashMap>> with DashMap for concurrent access
- Add unified AppError enum with thiserror
- Add configurable CLI paths (--my-cars, --cars-db, --times-dir, --tracks-db)
- Add serde deserialization for Car struct
- Add CSV escaping for fields with commas/quotes/newlines
- Fix track name splitting: split_once -> rsplit_once for _a handling
- Add integration tests, proptest property-based tests, and unit tests
- Add rustfmt.toml configuration
- Update documentation (API, ARCHITECTURE, DATA_STRUCTURES, README)

src/commands/fetch.rs

@@ -0,0 +1,48 @@
+//! Fetch command implementation.
+//!
+//! Downloads car database, tracks database, and per-car timing data
+//! from remote repositories.
+
+use top_drives::{AppError, get_file};
+
+use crate::parsing::fetch_times;
+
+/// Base URL for the remote data repository on GitHub.
+/// Single place to update if the repository path or branch changes.
+const GITHUB_RAW_BASE: &str =
+    "https://github.com/TiagoXavi/topdrives-records/raw/refs/heads/main/src/database";
+
+/// Returns the full URL for the car database file.
+fn cars_db_url() -> String {
+    format!("{GITHUB_RAW_BASE}/cars_final.json")
+}
+
+/// Returns the full URL for the tracks database file.
+fn tracks_db_url() -> String {
+    format!("{GITHUB_RAW_BASE}/tracks_repo.json")
+}
+
+/// Downloads the tracks database file from the remote repository.
+async fn get_tracks_file(tracks_db_path: &str) -> Result<(), AppError> {
+    println!("Fetching tracks file...");
+    get_file(&tracks_db_url(), tracks_db_path).await
+}
+
+/// Downloads the cars database file from the remote repository.
+pub async fn get_cars_file(cars_db_path: &str) -> Result<(), AppError> {
+    println!("Fetching cars file...");
+    get_file(&cars_db_url(), cars_db_path).await
+}
+
+/// Executes the fetch command: downloads all data files.
+///
+/// Downloads the car database, tracks database, and per-car timing files.
+/// Fails fast on critical dependency failures — the cars DB must be available
+/// before per-car timing files can be fetched.
+pub async fn run_fetch(cars_db_path: &str, tracks_db_path: &str, times_dir: &str) -> Result<(), AppError> {
+    get_cars_file(cars_db_path).await?;
+    get_tracks_file(tracks_db_path).await?;
+    fetch_times(cars_db_path, times_dir).await?;
+    println!("Fetch complete — all files downloaded successfully.");
+    Ok(())
+}