fix: address review findings in grid sampler, parallel optimization, and CSV export

- Rename stale "GridSearchSampler" panic message to "GridSampler"
- Assert concurrency > 0 in optimize_parallel to prevent deadlock
- Fix inaccurate comment in CSV export (empty for all non-complete trials, not just pruned)

src/study/export.rs

@@ -91,7 +91,7 @@ where
         for trial in trials.iter() {
             write!(writer, "{}", trial.id)?;
 
-            // Value: empty for pruned trials.
+            // Value: empty for non-complete trials.
             if trial.state == TrialState::Complete {
                 write!(writer, ",{}", trial.value)?;
             } else {