Make BucketEntries::iter, PercentileValuesVecEntry fields, and TopNComputer::threshold public (#2890)

These items need to be accessible from the tantivy-datafusion crate:
- BucketEntries::iter() for iterating aggregation bucket results
- PercentileValuesVecEntry.key/.value for reading percentile results
- TopNComputer.threshold for Block-WAND score pruning in the inverted index provider

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Paul Masurel <paul@quickwit.io>

src/aggregation/agg_result.rs

@@ -208,7 +208,8 @@ pub enum BucketEntries<T> {
 }
 
 impl<T> BucketEntries<T> {
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = &'a T> + 'a> {
+    /// Iterate over all bucket entries.
+    pub fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = &'a T> + 'a> {
         match self {
             BucketEntries::Vec(vec) => Box::new(vec.iter()),
             BucketEntries::HashMap(map) => Box::new(map.values()),

src/aggregation/metric/mod.rs

@@ -107,10 +107,9 @@ pub enum PercentileValues {
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 /// The entry when requesting percentiles with keyed: false
 pub struct PercentileValuesVecEntry {
-    /// Percentile
+    /// The percentile key (e.g. 1.0, 5.0, 25.0).
     pub key: f64,
-
-    /// Value at the percentile
+    /// The percentile value. `NaN` when there are no values.
     pub value: f64,
 }
 

src/collector/top_score_collector.rs

@@ -513,7 +513,9 @@ pub struct TopNComputer<Score, D, C> {
     /// The buffer reverses sort order to get top-semantics instead of bottom-semantics
     buffer: Vec<ComparableDoc<Score, D>>,
     top_n: usize,
-    pub(crate) threshold: Option<Score>,
+    /// The current threshold for pruning. Documents with scores at or below
+    /// this value are skipped by `push()`. Updated when the buffer is truncated.
+    pub threshold: Option<Score>,
     comparator: C,
 }