Fix bare Fn shortcut detection on macOS

[marcellaholm]

Summary

• Detect bare Fn hold-to-transcribe from the secondary-Fn flag transition instead of requiring the event keycode to be kVK_Function
• Let unrelated flagsChanged events pass through while Fn state is unchanged

Tests

• swift test --package-path macos/AgentCLI

[basnijholt]

Awesome!