Add async scanning with asyncio for faster port scans

[stabrea]

Summary

Replace the current threading-based port scanning with Python asyncio for significantly faster and more scalable network scanning.

Motivation

• Threading in Python is limited by the GIL and has high overhead per-thread
• asyncio with non-blocking sockets can handle thousands of concurrent connections
• Modern Python (3.11+) has significant asyncio performance improvements
• Scan times for large port ranges (1-65535) can drop from minutes to seconds

Proposed Approach

1. Async scanner module:

   • Use asyncio.open_connection() for TCP connect scans
   • Semaphore-based concurrency limiting (configurable, default 1000)
   • Timeout handling per-connection with asyncio.wait_for()

2. SYN scan support:

   • Raw socket async SYN scanning (requires root/sudo)
   • Fallback to connect scan when privileges unavailable

3. Progress reporting:

   • Async progress callback for real-time scan updates
   • ETA calculation based on scan rate

4. Backward compatibility:

   • Keep existing synchronous API as wrapper
   • Add --async flag to CLI for opt-in

Benchmarks to Target

Scan Type	Current (threading)	Target (asyncio)
Top 1000 ports	~30s	<5s
Full 65535 ports	~10min	<45s

Acceptance Criteria

• Async scanner with configurable concurrency
• At least 5x speedup on full port range scans
• Progress reporting during long scans
• Existing CLI interface preserved with async option