feat: add Storage trait and JSONL journal backend

Replace the internal Vec<CompletedTrial<V>> with a pluggable Storage<V>
trait. MemoryStorage is the default (no behavior change for existing
users). Behind the `journal` feature flag, JournalStorage persists
trials to a JSONL file with fs2 file locking for multi-process safety.

- Storage<V> trait with push(), trials_arc(), refresh() methods
- MemoryStorage<V> wraps Arc<RwLock<Vec<CompletedTrial<V>>>>
- JournalStorage<V> appends JSON lines with exclusive file locks
- Study::with_sampler_and_storage() general constructor
- Study::with_journal() convenience constructor (journal feature)
- Refresh from storage on create_trial() for multi-process discovery
- MSRV bumped to 1.89

src/error.rs

@@ -93,6 +93,11 @@ pub enum Error {
     #[cfg(feature = "async")]
     #[error("async task error: {0}")]
     TaskError(String),
+
+    /// Returned when a storage operation fails.
+    #[cfg(feature = "journal")]
+    #[error("storage error: {0}")]
+    Storage(String),
 }
 
 pub type Result<T> = core::result::Result<T, Error>;