Handle empty DacsByte length safely

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+- Prevent panic in `DacsByte::len` by handling empty level lists gracefully.
 - Embedded section handles in `BitVectorData` and added `BitVectorDataMeta` with
   `Serializable` support for both `BitVectorData` and `BitVector`, enabling
   zero-copy reconstruction from arena metadata.

INVENTORY.md

@@ -52,3 +52,4 @@
 ## Discovered Issues
 - `katex.html` performs manual string replacements; consider DOM-based manipulation.
 - Revisit zero-copy storage strategy: avoid extra copies when storing serialized bytes in structures.
+- Enforce that `DacsByte` always retains at least one level instead of relying on defensive length checks.

src/int_vectors/dacs_byte.rs

@@ -300,7 +300,7 @@ impl<I: BitVectorIndex> DacsByte<I> {
     /// Gets the number of integers.
     #[inline(always)]
     pub fn len(&self) -> usize {
-        self.data[0].len()
+        self.data.first().map_or(0, |lvl| lvl.len())
     }
 
     /// Checks if the vector is empty.