Write: NEAR Cross-Contract Calls Tutorial

Description

## Overview
Write a tutorial on NEAR cross-contract calls - promises, callbacks, and async patterns in smart contracts.

## Why This Matters
Cross-contract calls are essential but tricky. A clear tutorial reduces bugs and confusion.

## Content Requirements
- Promise basics
- Callback handling
- Error handling patterns
- Gas attachment
- Testing cross-contract calls

## Deliverables
1. Step-by-step tutorial
2. Working code examples
3. Common pitfalls section
4. Published and indexed