feat: add --json flag for structured output

[nightowlnerd]

Summary

• Add --json flag for machine-readable output with server stats (qps, success_rate, latency_p50_ms)
• Fix duplicate --- separator in progress output
• Fix "servers": null → [] for empty results
• Add minimal feedback when --progress=false

Test plan

• go test ./... passes
• ./dnscan --json --country ir --mode list --domain t.example.com outputs valid JSON
• JSON to stdout, progress to stderr (pipeable)