feat(engine): implement single-stream fallback engine for non-Range servers

[archittmittal]

Objective & Scope

Implement single-stream fallback downloading in src/download_engine.py.

Technical Details & Implementation Plan

• Streaming single-stream downloader when server lacks Accept-Ranges support
• Real-time streaming hash verification during single-stream transfer

Deliverable Files

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

Acceptance Criteria & Checkpoints

• Fallback triggered automatically when Accept-Ranges is absent

Parent Epic

Part of Epic #39