Document architecture redesign and add partition layer accessor

Documents a proposed redesign for cross-partition batch resolution, shifting trigger logic to per-destination accumulator thresholds and introducing entropy-based pruning criteria. Adds an `n_layers_per_partition` method to the index, exposing partition metadata with consistent error handling and clarified documentation regarding build-time structural properties.

src/obikindex/src/index.rs

@@ -142,6 +142,19 @@ impl KmerIndex {
     pub fn minimizer_size(&self) -> usize { self.meta.config.minimizer_size }
     pub fn n_partitions(&self) -> usize { self.partition.n_partitions() }
 
+    /// Number of layers per partition.
+    ///
+    /// Structural property of the index, fixed at build time and
+    /// homogeneous across all partitions — reading it off partition 0
+    /// is enough, no need to scan every partition.
+    pub fn n_layers_per_partition(&self) -> OKIResult<usize> {
+        use obilayeredmap::meta::PartitionMeta;
+        let index_dir = self.partition.part_dir(0).join("index");
+        let meta = PartitionMeta::load(&index_dir)
+            .map_err(|e| OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
+        Ok(meta.n_layers)
+    }
+
     /// Expose the inner partition so the caller can run scatter into it.
     /// Call `mark_scattered` once scatter is complete.
     pub fn partition_mut(&mut self) -> &mut KmerPartition {