feat: add Pruner trait and NopPruner default implementation

Introduce the pruning extension point for the trial pruning system.
The Pruner trait allows deciding whether to stop a trial early based
on intermediate values. NopPruner (never prunes) is the default.

src/pruner/mod.rs

@@ -0,0 +1,60 @@
+//! Pruner trait and implementations for trial pruning.
+//!
+//! Pruners decide whether to stop (prune) a trial early based on its
+//! intermediate values compared to other trials. This is useful for
+//! discarding unpromising trials before they complete, saving compute.
+
+mod nop;
+
+pub use nop::NopPruner;
+
+use crate::sampler::CompletedTrial;
+
+/// Trait for pluggable trial pruning strategies.
+///
+/// Pruners are consulted after each intermediate value is reported to
+/// decide whether the trial should be stopped early. The trait requires
+/// `Send + Sync` to support concurrent and async optimization.
+///
+/// # Implementing a custom pruner
+///
+/// ```
+/// use optimizer::pruner::Pruner;
+/// use optimizer::sampler::CompletedTrial;
+///
+/// struct MyPruner {
+///     threshold: f64,
+/// }
+///
+/// impl Pruner for MyPruner {
+///     fn should_prune(
+///         &self,
+///         _trial_id: u64,
+///         _step: u64,
+///         intermediate_values: &[(u64, f64)],
+///         _completed_trials: &[CompletedTrial],
+///     ) -> bool {
+///         // Prune if the latest value exceeds the threshold
+///         intermediate_values
+///             .last()
+///             .is_some_and(|&(_, v)| v > self.threshold)
+///     }
+/// }
+/// ```
+pub trait Pruner: Send + Sync {
+    /// Decide whether to prune a trial at the given step.
+    ///
+    /// # Arguments
+    ///
+    /// * `trial_id` - The current trial's ID.
+    /// * `step` - The step at which the intermediate value was reported.
+    /// * `intermediate_values` - All `(step, value)` pairs reported so far for this trial.
+    /// * `completed_trials` - History of all completed trials (for comparison).
+    fn should_prune(
+        &self,
+        trial_id: u64,
+        step: u64,
+        intermediate_values: &[(u64, f64)],
+        completed_trials: &[CompletedTrial],
+    ) -> bool;
+}

src/pruner/nop.rs

@@ -0,0 +1,17 @@
+use super::Pruner;
+use crate::sampler::CompletedTrial;
+
+/// A pruner that never prunes. This is the default when no pruner is configured.
+pub struct NopPruner;
+
+impl Pruner for NopPruner {
+    fn should_prune(
+        &self,
+        _trial_id: u64,
+        _step: u64,
+        _intermediate_values: &[(u64, f64)],
+        _completed_trials: &[CompletedTrial],
+    ) -> bool {
+        false
+    }
+}