feat(engine): implement core httpx async range download engine and worker pool

[archittmittal]

Objective & Scope

Implement the core asynchronous range download engine in src/download_engine.py.

Technical Details & Implementation Plan

• httpx.AsyncClient connection pooling with HTTP/2 multiplexing
• HEAD probe capability detector (Accept-Ranges, Content-Length, ETag)
• Coroutine worker pool bounded by asyncio.Semaphore
• Graceful shutdown signal handler (SIGINT / Ctrl+C) saving state before exit

Deliverable Files

• src/download_engine.py
• tests/unit/test_download_engine.py

Acceptance Criteria & Checkpoints

• Parallel chunk downloads verified across test server
• Graceful shutdown saves state without data corruption

Parent Epic

Part of Epic #39