Add proxy protocol auto-detection

[francomano]

Description

Add a check that auto-detects the proxy protocol by probing the proxy server with SOCKS5, SOCKS4, and HTTP CONNECT greetings. Users often don't know what type of proxy they're behind, and misconfiguration is the #1 cause of "proxy doesn't work."

sing-box and xray-core auto-detect proxy types. proxychains-ng users frequently misconfigure proxy types (issue #246).

Codebase starting point

• core/adapters/socks.go — SOCKS4/SOCKS5 protocol implementations, shows greeting byte sequences
• core/adapters/http_proxy.go — HTTP/HTTPS CONNECT implementation
• core/adapters/factory.go — AdapterFactory switch on proxy type
• core/checks/register.go — where to register the new check

Acceptance Criteria

• Create core/checks/proxy_fingerprint/check.go implementing check.Checker
• Send a SOCKS5 greeting (05 01 00) and analyze response
• Send a SOCKS4 greeting and analyze response
• Send an HTTP CONNECT request and analyze response
• Identify which protocol the proxy actually speaks
• If proxy type was user-specified, validate it matches actual behavior
• Suggest the correct type if mismatch detected
• No dependencies (standalone check)
• Register in core/checks/register.go
• Add unit tests in core/checks/proxy_fingerprint/check_test.go
• Update README.md checks table