Step 2: Move allocate_leaf method to insert_operations

- Move allocate_leaf method from lib.rs to insert_operations.rs
- Remove original method from lib.rs
- All tests still pass (35/35)
- Working incrementally to avoid breaking changes

Author: KentBeck

rust/src/insert_operations.rs

@@ -8,8 +8,12 @@ use crate::error::{BPlusTreeError, BTreeResult, ModifyResult};
 use crate::types::{BPlusTreeMap, NodeRef, LeafNode, BranchNode, NodeId, InsertResult, SplitNodeData};
 use std::marker::PhantomData;
 
-// This module will contain INSERT operations - for now it's just a placeholder
-// We'll move methods here incrementally to avoid breaking the build
+impl<K: Ord + Clone, V: Clone> BPlusTreeMap<K, V> {
+    /// Allocate a new leaf node in the arena and return its ID.
+    pub fn allocate_leaf(&mut self, leaf: LeafNode<K, V>) -> NodeId {
+        self.leaf_arena.allocate(leaf)
+    }
+}
 
 #[cfg(test)]
 mod tests {

rust/src/lib.rs

@@ -1399,10 +1399,7 @@ impl<K: Ord + Clone, V: Clone> BPlusTreeMap<K, V> {
     // ENHANCED ARENA-BASED ALLOCATION FOR LEAF NODES
     // ============================================================================
 
-    /// Allocate a new leaf node in the arena and return its ID.
-    pub fn allocate_leaf(&mut self, leaf: LeafNode<K, V>) -> NodeId {
-        self.leaf_arena.allocate(leaf)
-    }
+    // allocate_leaf method moved to insert_operations.rs module
 
     /// Deallocate a leaf node from the arena.
     pub fn deallocate_leaf(&mut self, id: NodeId) -> Option<LeafNode<K, V>> {