feat: unify optimize and optimize_with via blanket Objective impl

- Add blanket `impl Objective<V> for Fn(&mut Trial) -> Result<V, E>`
  so closures work directly with `optimize`
- Rewrite optimize, optimize_async, optimize_parallel to accept
  `impl Objective<V>` with before_trial/after_trial hooks
- Remove optimize_with, optimize_with_async, optimize_with_parallel
- Remove max_retries and retry logic from Objective trait
- Add explicit closure type annotations for HRTB inference
- Convert FnMut test closures to Fn via RefCell/Cell

examples/multi_objective.rs

@@ -15,7 +15,7 @@ fn main() -> optimizer::Result<()> {
 
     // Classic bi-objective: f1(x) = x², f2(x) = (x-1)²
     // The Pareto front is the curve where improving f1 worsens f2.
-    study.optimize(50, |trial| {
+    study.optimize(50, |trial: &mut optimizer::Trial| {
         let xv = x.suggest(trial)?;
         let f1 = xv * xv;
         let f2 = (xv - 1.0) * (xv - 1.0);