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

.github/workflows/ci.yml

@@ -139,14 +139,14 @@ jobs:
           fail_ci_if_error: true
 
   msrv:
-    name: MSRV (1.88)
+    name: MSRV (1.89)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - name: Install Rust 1.88
+      - name: Install Rust 1.89
         run: |
-          rustup override set 1.88
-          rustup update 1.88
+          rustup override set 1.89
+          rustup update 1.89
       - uses: Swatinem/rust-cache@v2
       - name: Check compilation
         run: cargo check --all-features