Optimize dict array validation to use true_count() (#8263)

## Summary

Replace an inefficient `iter().all()` check with a direct `true_count()`
comparison when validating that all values in a dictionary array are
referenced.

---------

Signed-off-by: Claude <noreply@anthropic.com>

Author: joseph-isaacs

vortex-array/src/arrays/dict/array.rs

@@ -135,7 +135,7 @@ pub trait DictArrayExt: TypedArrayRef<Dict> + DictArraySlotsExt {
             }
 
             let referenced_mask = self.compute_referenced_values_mask(true)?;
-            let all_referenced = referenced_mask.iter().all(|v| v);
+            let all_referenced = referenced_mask.true_count() == referenced_mask.len();
 
             vortex_ensure!(all_referenced, "value in dict not referenced");
         }