perf(core): add parallel downloads for bulk installs

[CalvinAllen]

Implement parallel downloads for faster bulk installation operations.

Goal

When installing multiple runtime versions (e.g., via dtvem install with a runtimes.json file), download archives simultaneously instead of sequentially.

Performance Target: 3-5x faster for bulk installs

Implementation

• Implement parallel download pool with configurable concurrency
• Add --parallel flag or config option to control concurrent downloads
• Default to sensible concurrency (e.g., 3-4 simultaneous downloads)
• Show combined progress for parallel downloads

Areas to Investigate

• Download parallelization strategy (how many simultaneous downloads?)
• Progress bar UX for multiple concurrent downloads
• Error handling when one download fails mid-batch
• Rate limiting / bandwidth considerations

Related

• perf(shim): optimize shim startup time and binary size #72 - Shim-specific performance optimizations
• perf(ci): strip debug symbols from builds #73 - Binary size reduction (symbol stripping)
• refactor(shim): implement lite provider interface #74 - Lite provider interface