Add connection stability / long-lived test check

[francomano]

Description

Add a stability test that runs repeated probes through the proxy over time to detect intermittent failures, connection drops, and latency spikes.

Codebase starting point

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

Acceptance Criteria

• Create core/checks/stability_test/check.go implementing check.Checker
• Run configurable probes (default 20) at intervals (default 5s)
• Each probe: HTTP request through proxy, measure latency and success
• Report: success rate, avg/min/max latency, standard deviation
• Detect connection drops via consecutive failures then recovery
• Register in core/checks/register.go
• Add unit tests in core/checks/stability_test/check_test.go
• Update README.md checks table