Add optional aria2c download acceleration

[zeeshanhaque21]

Summary

• add an optional aria2c backend for parallel Hugging Face model pulls
• resume existing .incomplete files with 16 segmented connections and four concurrent files
• report accurate structured progress through aria2 JSON-RPC instead of sparse-file size
• verify Hugging Face SHA-256 checksums before atomic rename
• keep Hugging Face credentials off process arguments and fall back to Python when aria2c is unavailable

Usage

Install aria2 with Homebrew, then use the default auto selection or pass --download-backend aria2 to require it. Pass --download-backend python to retain the built-in path.

Verification

• repository CI test set plus aria2, Hugging Face loader, and artifact suites
• real loopback aria2 resume from a 512 KiB partial with final SHA-256 verification
• Python bytecode compilation

Closes #451