feat: add download manager with pause/resume and crash recovery

[abdulroufsidhu]

Add a fully-featured async download manager supporting:

• Concurrent downloads with configurable worker pool
• Pause and resume functionality with state persistence
• Automatic crash recovery and resume capability
• Progress tracking with event subscriptions
• Retry logic with exponential backoff
• Multiple download backends (reqwest with streaming) New modules:
• download/manager.rs - Main coordinator
• download/worker.rs - Worker pool management
• download/queue.rs - Task queue with persistence
• download/backend.rs - HTTP download implementation
• download/progress.rs - Progress tracking
• download/retry.rs - Retry policies
• download/events.rs - Event system
• download/types.rs - Core types and state machine Add examples:
• basic_usage.rs - Simple download example
• pause_resume.rs - Pause/resume demonstration
• crash_recovery.rs - Crash recovery testing Update dependencies for tonic/prost compatibility.