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

optimizer-derive/Cargo.toml

@@ -2,7 +2,7 @@
 name = "optimizer-derive"
 version = "0.1.0"
 edition = "2024"
-rust-version = "1.88"
+rust-version = "1.89"
 license = "MIT"
 description = "Derive macros for the optimizer crate"
 repository = "https://github.com/raimannma/rust-optimizer"