Implement partially lock-free tree with delayed balancing

[konard]

Summary

This PR implements a partially lock-free size balanced tree with delayed balancing to address issue #78.

Key Features

✅ Lock-free insertions: Multiple insertions (N) can be performed concurrently using ReaderWriterLockSlim read locks
✅ Delayed balancing: Balancing operations are delayed until a configurable threshold of insertions is reached
✅ Thread-safe operations: Uses atomic operations (Interlocked) for tracking insertion counts
✅ Exclusive locking for balancing: Tree balancing only locks when necessary using write locks

Implementation Details

Core Components

• PartiallyLockFreeSizeBalancedTreeMethods<TElement>: Abstract base class extending SizeBalancedTreeMethods
• Configurable balancing threshold: Default 10 insertions before balancing triggers
• Manual balancing trigger: ForceBalancing() method for explicit balancing control
• Thread-safe counters: Atomic operations for tracking insertion counts
• Custom delayed balancing logic: Mirrors original maintain operations with delayed execution

Thread Safety Strategy

1. Read locks for insertions: Multiple threads can insert concurrently using ReaderWriterLockSlim.EnterReadLock()
2. Write locks for balancing: Exclusive write lock during balancing operations
3. Double-check pattern: Prevents unnecessary balancing operations in concurrent scenarios
4. Atomic counters: Interlocked operations ensure thread-safe insertion counting

Test Coverage

• ✅ Basic attach/detach operations
• ✅ Random insertion/deletion patterns
• ✅ Delayed balancing threshold verification
• ✅ Manual balancing trigger functionality
• ✅ Concurrent insertion stress testing (10 threads × 20 insertions each)

Performance Benefits

• Reduced contention: N insertions can proceed without blocking on balancing
• Improved throughput: Concurrent read operations during insertion phases
• Configurable balance: Threshold can be tuned based on usage patterns
• Graceful degradation: Falls back to locked balancing when needed

Usage Example

// Create tree with balancing threshold of 5 insertions
var tree = new PartiallyLockFreeSizeBalancedTree<uint>(capacity: 1000, balancingThreshold: 5);

// Insert elements - first 4 insertions are lock-free
for (uint i = 1; i <= 4; i++)
{
    var node = tree.Allocate();
    tree.Attach(ref tree.Root, node);  // Lock-free insertion
}

// 5th insertion triggers delayed balancing with exclusive lock
var finalNode = tree.Allocate();  
tree.Attach(ref tree.Root, finalNode);  // Triggers balancing

// Manual balancing trigger
tree.ForceBalancing();

Fixes #78

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