feature: streamlined tansfer

[nullable-eth]

Streamline Transfer Architecture and Performance

Overview

Refactored the transfer package to provide a unified, high-performance file transfer system with intelligent optimization strategies.

Key Improvements

🏗️ Unified Transfer Architecture

• Introduced TransferClient as a unified interface that handles common logic (file existence checks, size validation, logging)
• Clean separation between public FileTransferrer interface and internal internalTransferer implementations
• Automatic transfer method selection with fallback capabilities

⚡ Performance Optimizations

• Rsync: Intelligent batch transfers, configurable compression levels, parallel streams, checksum skipping for trusted networks
• SCP/SFTP: Optimized buffer sizes (1MB), concurrent transfer limits, persistent SSH connections
• Smart file size checking to avoid unnecessary re-transfers of identical files

🧠 Intelligent Transfer Logic

• Pre-transfer validation: checks file existence and sizes before initiating transfers
• Batch optimization: groups files by directory for more efficient rsync operations
• Transfer skipping: automatically skips files with identical sizes to reduce bandwidth usage

📊 Enhanced Logging & Monitoring

• Structured logging with transfer events (started/completed/skipped)
• Detailed transfer metrics including file sizes, durations, and transfer reasons
• Improved error reporting with context-rich debug information

🔧 Code Quality

• Added comprehensive package documentation
• Consistent error handling patterns across all transfer methods
• Maintained backward compatibility with existing interfaces

Technical Details

• Rsync optimizations: --inplace, --partial, configurable compression, SSH connection reuse
• SFTP optimizations: Large transfer buffers, connection pooling, concurrent transfer management
• Smart batching: Files grouped by source directory for efficient multi-file transfers

This refactor significantly improves transfer performance while maintaining reliability and adding intelligent optimization strategies.