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

Cargo.toml

@@ -5,7 +5,7 @@ members = ["optimizer-derive"]
 name = "optimizer"
 version = "0.8.1"
 edition = "2024"
-rust-version = "1.88"
+rust-version = "1.89"
 license = "MIT"
 authors = ["Manuel Raimann <raimannma@outlook.de"]
 description = "A Rust library for optimization algorithms."
@@ -26,12 +26,14 @@ serde_json = { version = "1", optional = true }
 tracing = { version = "0.1.29", optional = true }
 sobol_burley = { version = "0.5", optional = true }
 nalgebra = { version = "0.34", optional = true }
+fs2 = { version = "0.4", optional = true }
 
 [features]
 default = []
 async = ["dep:tokio"]
 derive = ["dep:optimizer-derive"]
 serde = ["dep:serde", "dep:serde_json"]
+journal = ["dep:fs2", "serde"]
 tracing = ["dep:tracing"]
 sobol = ["dep:sobol_burley"]
 cma-es = ["dep:nalgebra"]
@@ -71,3 +73,7 @@ required-features = ["derive"]
 name = "visualization_demo"
 path = "examples/visualization_demo.rs"
 required-features = ["visualization"]
+
+[[test]]
+name = "journal_tests"
+required-features = ["journal"]