feat: Integrate solana-snapshot-finder-go for intelligent snapshot source discovery

[7layermagik]

Summary

Integrates the solana-snapshot-finder-go library to provide intelligent snapshot source discovery and selection. This replaces the previous snapshot download logic with a two-stage speed testing algorithm that finds the fastest available snapshot sources.

Key Features

Intelligent Node Discovery

• Two-stage algorithm: fast parallel triage followed by sustained speed testing
• Filters nodes by Solana version, RTT, TCP connectivity, and snapshot age
• Ranks nodes by actual download speed, not just network latency
• Configurable fallback to try multiple ranked nodes if the best one fails

HTTP Streaming Mode

• Stream snapshots directly from HTTP to the processing pipeline
• Optional disk saving while streaming using io.TeeReader (parallel processing + save)
• When save_to_disk=false (default): no disk space required for snapshot files
• When save_to_disk=true: saves snapshots to download_path while streaming

Smart Incremental Snapshot Selection

• Prioritizes freshness (end slot) over pure speed for incrementals
• Tries the same source as the full snapshot first (fastest path)
• Falls back to cluster-wide search if the primary source doesn't have a matching incremental
• Minimum speed threshold (min_incremental_speed_mbs) to filter out slow nodes

Retry and Cleanup Logic

• Automatic retry with re-discovery when incremental download fails mid-way
• Partial download cleanup on Ctrl+C or errors (no orphaned files)
• Up to 3 retry attempts with fresh source discovery between attempts

Configuration

New [snapshot] section in mithril.toml with comprehensive options:

[snapshot]
    # Save snapshots to disk while streaming
    save_to_disk = false
    download_path = "/data/snapshots"
    
    # Stage 1: Fast parallel triage
    stage1_warm_kib = 512
    stage1_window_kib = 512
    stage1_windows = 4
    stage1_timeout_ms = 3000
    
    # Stage 2: Sustained speed test
    stage2_top_k = 8
    stage2_warm_sec = 2
    stage2_measure_sec = 2
    stage2_min_ratio = 0.6
    
    # Node filtering
    max_rtt_ms = 200
    min_node_version = "2.2.0"
    
    # Fallback resilience
    max_snapshot_url_attempts = 3

See docs/snapshot.md for complete documentation.

Dependencies

Requires the updated solana-snapshot-finder-go library with library mode support (PR #6).

Test plan

• Run verify-live with default config (HTTP streaming, no disk save)
• Run with save_to_disk = true to verify parallel save works
• Test Ctrl+C during download to verify cleanup
• Verify Stage 1 and Stage 2 logging output
• Test incremental retry by simulating a mid-download failure