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/user_data.rs

@@ -0,0 +1,99 @@
+//! User data loading from `my_cars.json`.
+//!
+//! Handles parsing the game export file to extract owned car GUIDs
+//! and wishlist entries.
+
+use std::collections::HashSet;
+use std::fs::File;
+use std::io::BufReader;
+
+use serde_json::Value;
+
+use crate::types::AppError;
+
+/// Parsed user data from `my_cars.json`
+///
+/// Holds both the owned car GUIDs and wishlist GUIDs extracted from a single
+/// file read, avoiding duplicate I/O.
+#[derive(Debug, Default)]
+pub struct MyCarsData {
+    /// GUIDs of cars the user owns (from `playerDeck[].cardId` where `locked == true`)
+    pub owned: HashSet<String>,
+    /// GUIDs of wishlisted cars (from `wishlists[0].cards`)
+    pub wishlist: HashSet<String>,
+}
+
+/// Loads and parses `my_cars.json` once, extracting both owned and wishlist data.
+///
+/// Opens the game export file a single time and extracts:
+/// - **Owned cars** from `playerDeck[].cardId` (filtered by `locked == true`)
+/// - **Wishlist** from `wishlists[0].cards`
+///
+/// # File Source
+///
+/// * **File**: `my_cars.json` (exported from Top Drives game)
+///
+/// # Error Handling
+///
+/// Returns an `AppError` if the file cannot be opened or parsed.
+/// Use `MyCarsData::default()` for graceful degradation when the file is optional.
+pub fn load_my_cars_data(path: &str) -> Result<MyCarsData, AppError> {
+    let file = File::open(path).map_err(|e| {
+        if e.kind() == std::io::ErrorKind::NotFound {
+            AppError::MissingFile(format!("{path}: {e}"))
+        } else {
+            AppError::Io(e)
+        }
+    })?;
+    let reader = BufReader::new(file);
+    let json: Value = serde_json::from_reader(reader)
+        .map_err(|e| AppError::Parse { file: path.to_string(), reason: e.to_string() })?;
+
+    // Extract owned car GUIDs from playerDeck
+    let owned: HashSet<String> = json
+        .get("playerDeck")
+        .and_then(|d| d.as_array())
+        .map(|deck| {
+            deck.iter()
+                .filter_map(|card| {
+                    if card.get("locked").and_then(|v| v.as_bool()).unwrap_or(false) {
+                        card.get("cardId").and_then(|v| v.as_str()).map(|s| s.to_string())
+                    } else {
+                        None
+                    }
+                })
+                .collect()
+        })
+        .unwrap_or_default();
+
+    // Extract wishlist GUIDs from wishlists[0].cards
+    let wishlist: HashSet<String> = json
+        .get("wishlists")
+        .and_then(|w| w.get(0))
+        .and_then(|w| w.get("cards"))
+        .and_then(|c| c.as_array())
+        .map(|cards| {
+            cards
+                .iter()
+                .filter_map(|c| c.as_str().map(|s| s.to_string()))
+                .collect()
+        })
+        .unwrap_or_default();
+
+    Ok(MyCarsData { owned, wishlist })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_load_my_cars_data_missing_file() {
+        let result = load_my_cars_data("nonexistent_file.json");
+        assert!(result.is_err());
+        match result.unwrap_err() {
+            AppError::MissingFile(msg) => assert!(msg.contains("nonexistent_file.json")),
+            other => panic!("Expected MissingFile error, got: {other}"),
+        }
+    }
+}