feat: implement macOS connectivity detection

[polw1]

Summary

Adds macOS support for connection_status() using Apple’s Network framework.

• implement the macOS backend with NWPathMonitor
• map connectivity, metered, constrained, and transport status into the existing plugin types
• treat only nw_path_status_satisfied as connected: true
• document the macOS path-status semantics in code comments and README
• add test coverage for macOS path-status handling and connection-type priority

Closes #8

Test plan

• cargo check
• cargo test --workspace --lib
• npm run standards

[polw1]

Creating and destroying an NWPathMonitor on every call feels fragile, especially with a fixed timeout. Apple’s API delivers path updates through callbacks scheduled on a dispatch queue, so treating the first frontend request as a synchronous one-shot query felt like the wrong model.

NWPathMonitor is better treated as a long-lived monitor than as a synchronous status getter.

So i changed the implementation to create the monitor once, caches the latest status, and returns that cached value from plugin calls.

I also start the monitor during plugin setup so the system has a chance to receive the first path update before the frontend makes its first real request, instead of making that request responsible for initializing the monitor.