Add resumable model download with retry, timeout, and offline mode

[janhilgard]

Summary

• Adds a pre-download step with configurable retry (exponential backoff) and timeout before load_model() is called, so interrupted downloads of large models can be resumed
• New CLI flags for serve: --download-timeout, --download-retries, --offline
• New standalone subcommand: vllm-mlx download <model> for pre-warming HF caches (useful for CI/CD)
• Replaces direct snapshot_download() call in tokenizer fallback path with the new retry-aware wrapper

Motivation

Addresses #75 — HuggingFace downloads hang or fail around 10GB for large models with no way to resume.

Usage

# Download model to cache without starting server
vllm-mlx download mlx-community/Qwen3-Next-80B-A3B-Instruct-6bit

# Serve with custom retry/timeout
vllm-mlx serve <model> --download-timeout 600 --download-retries 5

# Offline mode (only locally cached models)
vllm-mlx serve <model> --offline

Test plan

• 12 unit tests pass (pytest tests/test_download.py -v)
• Manual test: vllm-mlx download mlx-community/Qwen3-0.6B-4bit succeeds
• Manual test: nonexistent model fails with clear error message after retries
• ruff check and black pass on all changed files

🤖 Generated with Claude Code

[waybarrios]

@janhilgard
For next time, could you please organize your commits a bit better? Having so many commits in a single PR makes it difficult to review the changes. I recommend squashing them all into one commit for this and future PRs

[janhilgard]

You're right, sorry about that! I've squashed everything into a single clean commit now.