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/lib.rs

@@ -233,6 +233,7 @@ pub mod pareto;
 pub mod pruner;
 mod rng_util;
 pub mod sampler;
+pub mod storage;
 mod study;
 mod trial;
 mod types;
@@ -269,6 +270,9 @@ pub use sampler::random::RandomSampler;
 #[cfg(feature = "sobol")]
 pub use sampler::sobol::SobolSampler;
 pub use sampler::tpe::TpeSampler;
+#[cfg(feature = "journal")]
+pub use storage::JournalStorage;
+pub use storage::{MemoryStorage, Storage};
 pub use study::Study;
 #[cfg(feature = "serde")]
 pub use study::StudySnapshot;
@@ -314,6 +318,9 @@ pub mod prelude {
     #[cfg(feature = "sobol")]
     pub use crate::sampler::sobol::SobolSampler;
     pub use crate::sampler::tpe::TpeSampler;
+    #[cfg(feature = "journal")]
+    pub use crate::storage::JournalStorage;
+    pub use crate::storage::{MemoryStorage, Storage};
     pub use crate::study::Study;
     #[cfg(feature = "serde")]
     pub use crate::study::StudySnapshot;