feat: add iteration and aggregation to compact int vec

Implemented `sum()`, `count_nonzero()`, and `iter()` to complete the numeric vector interface. The builder now computes aggregate values across memory-mapped regions and overflow entries, while the reader delegates these operations to its inherent methods. The iterator provides zero-copy access to underlying `u32` elements.

src/obicompactvec/src/builder.rs

@@ -5,7 +5,7 @@ use std::path::{Path, PathBuf};
 
 use memmap2::MmapMut;
 
-use crate::format::{HEADER_SIZE, finalize_pciv, parse_overflow_entry};
+use crate::format::{byte_count_nonzero, byte_sum, HEADER_SIZE, finalize_pciv, parse_overflow_entry};
 use crate::reader::PersistentCompactIntVec;
 
 pub struct PersistentCompactIntVecBuilder {
@@ -148,6 +148,12 @@ impl IntSlice for PersistentCompactIntVecBuilder {
     fn overflow_entries(&self) -> impl Iterator<Item = (usize, u32)> + '_ {
         self.overflow.iter().map(|(&k, &v)| (k, v))
     }
+    fn sum(&self) -> u64 {
+        byte_sum(&self.mmap[HEADER_SIZE..HEADER_SIZE + self.n], self.overflow.values().copied())
+    }
+    fn count_nonzero(&self) -> u64 {
+        byte_count_nonzero(&self.mmap[HEADER_SIZE..HEADER_SIZE + self.n])
+    }
 }
 
 impl IntSliceMut for PersistentCompactIntVecBuilder {

src/obicompactvec/src/memoryintvec.rs

@@ -80,6 +80,7 @@ impl IntSlice for MemoryIntVec {
     fn overflow_entries(&self) -> impl Iterator<Item = (usize, u32)> + '_ {
         self.overflow.iter().map(|(&k, &v)| (k, v))
     }
+    fn iter(&self) -> impl Iterator<Item = u32> + '_ { self.iter() }
     fn sum(&self) -> u64 { self.sum() }
     fn count_nonzero(&self) -> u64 { self.count_nonzero() }
 }

src/obicompactvec/src/reader.rs

@@ -363,6 +363,9 @@ impl IntSlice for PersistentCompactIntVec {
     fn overflow_entries(&self) -> impl Iterator<Item = (usize, u32)> + '_ {
         (0..self.n_overflow).map(|i| (self.data_slot(i), self.data_value(i)))
     }
+    fn iter(&self) -> impl Iterator<Item = u32> + '_ { self.iter() }
+    fn sum(&self) -> u64 { self.sum() }
+    fn count_nonzero(&self) -> u64 { self.count_nonzero() }
 }
 
 impl<'a> IntoIterator for &'a PersistentCompactIntVec {