feat(engine): implement asynchronous range download engine, token bucket rate limiter, and CLI interface

[archittmittal]

Objective & Overview

Integrate all storage and algorithmic components into the core asynchronous download orchestrator, implement the token bucket rate limiter, and expose the user CLI via Click.

Technical Specification & Design

• Rate Limiting: Millisecond-resolution async Token Bucket bandwidth throttler.
• Async Orchestration: httpx.AsyncClient connection pooling, HTTP/2 multiplexing, HEAD probe capability detector.
• Signal Handling: SIGINT (Ctrl+C) graceful shutdown saving state before termination.
• CLI: Commands for download, resume, verify, status, manifest, clean with Rich progress bars.

Key Deliverables & Module Breakdown

• src/rate_limiter.py: Token bucket rate limiter
• src/download_engine.py: Async download engine and worker pool
• src/main.py: Click CLI interface and commands

Acceptance Criteria & Checkpoints

• Rate limiting accurate within $\pm 3%$ of limit
• Single-stream fallback mode functional
• CLI execution and graceful shutdown verified

Documentation References

• Architecture
• API Reference
• User Guide