test(benchmarks): implement automated benchmark harness comparing ReliaDL vs aria2, wget, and curl

[archittmittal]

Objective & Scope

Implement the comparative benchmark harness in tests/benchmarks/benchmark_runner.py.

Technical Details & Implementation Plan

• Benchmark execution engine comparing ReliaDL vs aria2, wget, and curl
• Capture metrics: Total Time, Goodput Speedup, Wasted Re-download %, Tail Latency (P99)
• Output automated Markdown benchmark tables

Deliverable Files

• tests/benchmarks/benchmark_runner.py
• tests/benchmarks/test_benchmarks.py

Acceptance Criteria & Checkpoints

• Benchmark harness produces verifiable comparison tables and performance assertions

Parent Epic

Part of Epic #44