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)

docs/DATA_STRUCTURES.md

@@ -235,15 +235,10 @@ const MAX_RQ: u8 = 113;
 
 **Car Database Processing:**
 ```rust
-// Parse cars_repo.json
-let cars_data: Vec<Value> = serde_json::from_str(&file_content)?;
-for car in cars_data {
-    let car = Car {
-        rid: car["rid"].as_str().unwrap_or(""),
-        brand: car["brand"].as_str().unwrap_or(""),
-        // ... other fields
-    };
-}
+// Parse cars_repo.json — uses serde derive for deserialization
+let cars: Vec<Car> = serde_json::from_str(&content)?;
+// Falls back to manual Value-based parsing if serde deserialization fails
+// (for backward compatibility with older JSON formats)
 ```
 
 **Timing Data Processing:**
@@ -303,8 +298,9 @@ BestCarRecord {
 
 **RQ Range Analysis:**
 - Test each car from its base RQ up to `MAX_RQ` (113)
-- Compare against existing best times
+- Compare against existing best times using `DashMap` entry API
 - Update records if better performance found
+- Uses `DashMap` for fine-grained concurrent access (no global lock)
 
 ## Data Quality Measures
 
@@ -345,8 +341,8 @@ BestCarRecord {
 ### Memory Usage
 
 **Large Data Structures**:
-- `BEST_CARS` HashMap: Contains all best records across RQ ranges
-- Individual timing files: Loaded and parsed sequentially
+- `AnalysisContext.best_cars` (`DashMap<TripletKey, Vec<BestCarRecord>>`): Contains all best records across RQ ranges
+- Individual timing files: Loaded and parsed sequentially via `spawn_blocking`
 - User collection: Small, loaded once
 
 **Optimization Strategies**: