fix: use atomic counter for temp paths in journal tests

On Windows, SystemTime::now().as_nanos() has ~15ms resolution,
causing parallel tests to generate identical file paths and
corrupt each other's data.

tests/journal_tests.rs

@@ -10,13 +10,14 @@ use optimizer::storage::{JournalStorage, Storage};
 use optimizer::{Direction, Study};
 
 fn temp_path() -> std::path::PathBuf {
+    use std::sync::atomic::{AtomicU64, Ordering};
+    static COUNTER: AtomicU64 = AtomicU64::new(0);
+
     let mut path = std::env::temp_dir();
     path.push(format!(
-        "optimizer_journal_test_{}.jsonl",
-        std::time::SystemTime::now()
-            .duration_since(std::time::UNIX_EPOCH)
-            .unwrap()
-            .as_nanos()
+        "optimizer_journal_test_{}_{}.jsonl",
+        std::process::id(),
+        COUNTER.fetch_add(1, Ordering::Relaxed)
     ));
     path
 }