feat: add constraint handling with feasibility-aware trial ranking

Add constraint support so that optimization problems with constraints
(e.g., "model size < 100MB") prefer feasible solutions. Convention:
constraint value <= 0.0 means feasible.

- Add constraint_values field to Trial with set_constraints/getter
- Add constraints field to CompletedTrial with is_feasible() method
- Propagate constraints through complete_trial/prune_trial
- Make best_trial() and top_trials() constraint-aware: feasible trials
  rank above infeasible, infeasible ranked by total violation

src/sampler/bohb.rs

@@ -178,6 +178,7 @@ impl BohbSampler {
                         intermediate_values: trial.intermediate_values.clone(),
                         state: trial.state,
                         user_attrs: trial.user_attrs.clone(),
+                        constraints: trial.constraints.clone(),
                     })
             })
             .collect()
@@ -396,6 +397,7 @@ mod tests {
             intermediate_values,
             state: TrialState::Complete,
             user_attrs: HashMap::new(),
+            constraints: Vec::new(),
         }
     }
 

src/sampler/mod.rs

@@ -41,6 +41,9 @@ pub struct CompletedTrial<V = f64> {
     pub state: TrialState,
     /// User-defined attributes stored during the trial.
     pub user_attrs: HashMap<String, AttrValue>,
+    /// Constraint values for this trial (<=0.0 means feasible).
+    #[cfg_attr(feature = "serde", serde(default))]
+    pub constraints: Vec<f64>,
 }
 
 impl<V> CompletedTrial<V> {
@@ -61,6 +64,7 @@ impl<V> CompletedTrial<V> {
             intermediate_values: Vec::new(),
             state: TrialState::Complete,
             user_attrs: HashMap::new(),
+            constraints: Vec::new(),
         }
     }
 
@@ -83,6 +87,7 @@ impl<V> CompletedTrial<V> {
             intermediate_values,
             state: TrialState::Complete,
             user_attrs,
+            constraints: Vec::new(),
         }
     }
 
@@ -127,6 +132,14 @@ impl<V> CompletedTrial<V> {
         })
     }
 
+    /// Returns `true` if all constraints are satisfied (values <= 0.0).
+    ///
+    /// A trial with no constraints is considered feasible.
+    #[must_use]
+    pub fn is_feasible(&self) -> bool {
+        self.constraints.iter().all(|&c| c <= 0.0)
+    }
+
     /// Gets a user attribute by key.
     #[must_use]
     pub fn user_attr(&self, key: &str) -> Option<&AttrValue> {