Potential fixes for 2 code quality findings (#35)

* Apply suggested fix to src/store/weight.rs from Copilot Autofix
* WeightStore invariant add to ensure total_weight is not less than entry.weight
* saturating_sub used to ensure no overflow 
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Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

Author: TKorr

src/store/weight.rs

@@ -501,7 +501,16 @@ where
     pub fn try_insert(&mut self, key: K, value: Arc<V>) -> Result<Option<Arc<V>>, StoreFull> {
         let new_weight = self.compute_weight(value.as_ref());
         if let Some(entry) = self.map.get_mut(&key) {
-            let next_total = self.total_weight - entry.weight + new_weight;
+            debug_assert!(
+                self.total_weight >= entry.weight,
+                "WeightStore invariant violated: total_weight ({}) is less than entry.weight ({})",
+                self.total_weight,
+                entry.weight
+            );
+            let next_total = self
+                .total_weight
+                .saturating_sub(entry.weight)
+                .saturating_add(new_weight);
             if next_total > self.capacity_weight {
                 return Err(StoreFull);
             }
@@ -551,6 +560,10 @@ where
     /// ```
     pub fn remove(&mut self, key: &K) -> Option<Arc<V>> {
         let entry = self.map.remove(key)?;
+        debug_assert!(
+            self.total_weight >= entry.weight,
+            "total_weight underflow in remove"
+        );
         self.total_weight = self.total_weight.saturating_sub(entry.weight);
         self.metrics.inc_remove();
         Some(entry.value)