Add kill switch / reconnection leak test

[francomano]

Description

Add a kill switch test that simulates network interruption and probes for real IP exposure during reconnection windows. No CLI tool does this.

Codebase starting point

• core/checks/public_ip/check.go -- template
• core/checks/register.go -- registration
• core/check/types.go -- CategoryLeakDetection

Acceptance Criteria

• Create core/checks/killswitch_test/check.go implementing check.Checker
• Record baseline direct IP
• Establish proxy connection, verify proxy IP
• Simulate network interruption (close/reopen connections rapidly)
• During reconnection, probe for real IP via multiple services simultaneously
• If real IP appears, flag as kill switch failure
• Use goroutines with tight timing to catch transient leaks
• Depends on public_ip
• Register in core/checks/register.go
• Add unit tests in core/checks/killswitch_test/check_test.go
• Update README.md checks table